Chimeric fusion proteins and virus like particles from birnavirus vp2

ABSTRACT

The field of the invention refers to chimeric Virus Like Particles (VLP) derived from Birnavirus chimeric VP2 protein. In particular, the invention refers to chimeric VP2 fusion proteins which incorporate insertions and/or substitutions with one or more amino acids or particular peptide of interest while maintaining the capacity to assemble in the form of VLP. The invention identifies particular insertion and/or substitutions sites within VP2 P loop regions and outside said P loop regions. The invention also incorporates methods for the identification of preferred insertion and substitution sites within VP2 for the incorporation of particular amino acids and peptides of interest. The resulting chimeric VLP are of interest in the design of therapeutic and prophylactic vaccines as well as in the design of drug delivery systems, carriers for DNA and RNA in gene therapy, as targeted agents, in the development of antitoxins, and as diagnostic reagents.

TECHNICAL FIELD

The field of the invention refers to chimeric Virus Like Particles (VLP) derived from the Birnavirus chimeric VP2 protein. In particular, the invention refers to chimeric VP2 fusion proteins which incorporate insertions and/or substitutions with one or more amino acids or particular peptide of interest while maintaining the capacity to assemble in the form of VLP. The invention identifies particular insertion and/or substitutions sites within VP2 P loop regions and outside said P loop regions. The invention also incorporates methods for the identification of preferred insertion and substitution sites within VP2 for the incorporation of particular amino acids and peptides of interest. The resulting chimeric VLP are of interest in the design of therapeutic and prophylactic vaccines as well as in the design of drug delivery systems, carriers for DNA and RNA in gene therapy, as targeted agents, in the development of antitoxins, and as diagnostic reagents.

BACKGROUND OF THE INVENTION

Virus Like Particles (VLP) are nanometric structures resulting from the assembly of structural viral proteins. These particles resemble the virus from which they were derived but lack viral nucleic acid and are therefore not infectious. Virus Like Particles (VLP) are preferred forms in the design of vaccines and in other applications in human health and diagnostics.

Vaccines most often incorporate VLP that are derived from the causative agents of the disease as is exemplified by Hepatitis B VLP useful in vaccination against Hepatitis. However, VLP may be made to incorporate unrelated/heterologous peptides relevant to disease. These chimeric VLP help in antigen presentation and in promoting an immune response in the receiving subject. An example being VLP formed by hepatitis B core and surface antigen fused to the Malaria or HCV epitope, respectively [Grgacic E. et al. (2006) Methods 40(1):60-65]. Maintenance of VLP structure is an essential feature in the design of these agents.

VLP three dimensional nanometric structures not only provide the means for incorporating antigens for their improved presentation to the immune system but are also useful in the design of drug delivery systems [Georgens C. et al. (2005) Current Pharmaceutical Biotech. 6(1):49-55], as carriers for DNA in gene therapy [Ou W C. et al. (2001) J. Med. Virol., 64(3):366-373; and Krauzewicz N. et al (2000) Gene Therapy 7(13)1094-1102], as targeted agents [Gleiter S, and Lilie H. (2001) Protein Science 10(2):434-444], in the development of antitoxins [Manayani D J. et al. (2007) PLoS Pathogens 3(10):1422-1431] and as diagnostic reagents [Martinez-Torrecuadrada J L. et al. (2000) Clinical Diagnostic Lab. Immunol. 7(4):645-651]. Again, maintenance of VLP structure is a common and essential feature in the design of these agents.

Commonly described VLP include those derived from Hepatitis B, Papilloma, Polyoma and other viruses. Other VLP described include those derived from Infectious Bursal Disease Virus (IBDV).

IBDV belongs to the Birnaviridae family and is the causative agent of Gumboro disease in poultry. Wild-type IBDV particles are icosahedral, with T=13 symmetry and a single protein shell formed by 260 trimers of the VP2 protein (37 kDa). The inner side of the VP2 shell appears to be supported by a scaffold formed by 200 trimers of the VP3 protein (29 kDa). It has been suggested that a third protein, VP4 (28 kDa), may also play a scaffolding role. In normal virus assembly, protein components result from the proteolytic processing of a larger polypeptide pVP2-VP4-VP3 precursor (109 kDa). This precursor undergoes auto-catalysis to release a 512 amino acid VP2 precursor (pVP2), VP4 and VP3 polypeptides. VP4 belongs to the Lon protease family and is responsible for the proteolytic cleavage while pVP2 and VP3 polypeptides are directly responsible for capsid assembly. A final cleavage of pVP2 at its C-terminal end gives rise to the mature 441 amino acid form of VP2 found in the virion [Da Costa B. et al. (2002) J. Virology 76(5):2393-2402]. VP2 proteins found in different IBDV strains have been reported to present a protein sequence homology of over 80%. VP2 proteins of other Birnaviridae share homologies with IBDV of 40% for aquatic Birnavirus and 30% for Drosophila Birnavirus [Coulibaly F. et al. (2005) Cell 25,120(6):761-772].

It has been found that expression in eukaryotic cells of the IBDV pVP2-VP4-VP3 polyprotein gives rise to the formation of icosahedral T=13 VLP that appear morphologically and biochemically indistinguishable from IBDV capsids and that this process does not require the presence of the viral genome or other proteins encoded by the viral genome, such as VP5 and VP1 [Martinez-Torrecuadrada J L. et al. (2001) J. Virology 75(22):10815-10828].

The ability of IBDV proteins to generate T=13 provides a versatile system for the incorporation of foreign peptides of interest relevant to human disease in the form of a vaccine. This is exemplified by Delmas B. et al. in WO02088339 in which Green Fluorescent Protein (GFP) is engineered as a C-terminal fusion to the precursor polyprotein pVP2-VP4-VP3-GFP to produce T=13 VLP in which GFP is fused to VP3 and presumably located inside the VLP. Similarly, Rodriguez Aguirre J F. et al. in WO2005071069 describe pVP2-VP3-X fusion proteins were a peptide of interest in vaccination (X) is fused to the C-terminus of VP3. Most likely constructs incorporating a peptide of interest fused to VP3 result in icosahedral VLP were the peptide of interest is sequestered within the T=13 particle.

More so, expression of VP2 in insect cells, in the absence of other IDBV proteins, has been found to result in the formation of smaller size iscosahedral T=1 VLP [Martinez-Torrecuadrada J L. et al. (2003) Vaccine 21(17-18):1952-1960]. It has been reported that the expression of VP2 fragments between 441 and 466 amino acids leads to the formation of icosahedral VLP, whereas the longer VP2 fragments between 466 and 501 amino acids tend to form tubular particles [Ruiz Caston J. et al. WO2005105834; and Saugar I. et al. (2005) Structure 13(7):1007-1117]. This has been exploited by Rodriguez Aguirre J F. et al. in WO2007009673 which describe the incorporation of peptides of interest (X) in T=1 VLP produced as VP2-X terminal fusion proteins. Recent reports however suggest that C-terminus fused peptides are not exposed on the VLP surface [Coulibaly F. et al. (2005) Cell 25,120(6):761-772; Lee C C. et al. 2006 J. Struct Biol. 155(1):74-86; and Garriga D. et al. (2006) J. Virol. 80(14):6895-6905] and that purification procedures carried out on C-terminal fusions of VP2 with Histidine residues with a metal ion affinity column are most likely mediated by naturally occurring Histidine residues within VP2 [Doong et al., (2007) Anal. Chem. 79(20):7654-7656]. Therefore, VP2 terminal fusions most likely result in the incorporation of peptides of interest in a sequestered form inside the T=1 VLP. Furthermore high sequence variability is found in the loops of the P domains named BC (AA 219-224), DE (AA 249-254), FG (AA 283-287) and HI (AA 315-324) which also appear to be the targets of neutralizing antibodies and harbour mutations in escape mutants indicating that these regions are immunogenic [Lee C C. et al. (2006) J. Struct Biology 155(1):74-86].

Therefore, to date, incorporation of peptides of interest in IBDV derived VLP, T=1 and T=13, has focused on VP2 and VP3 terminal fusions that most likely result in sequestration of the peptide of interest inside the VLP, as referred in Rodriguez-Aguirre J F. et al. WO2005071069, and suboptimal presentation to cells, cell surface receptors, soluble factors or diagnostic reagents.

Incorporation by means of insertion or substitution of the peptide of interest within VP2 represents improved alternatives to terminal fusions. In particular, improvements may result from surface exposure of the inserted sequences or the total or partial sequestration within the VLP structure of the inserted peptides. It is recognized that while surface exposure may be necessary for targeting against other biological entities such as cell surface receptors or soluble factors, total o partial sequestration may be desirable to avoid biological degradation or proteolysis or in eliciting a cellular immune response. Therefore, chimeric VLP in which peptides and amino acids of interest are incorporated in accordance to their intended biological activity could represent improved candidate vaccines, DNA or RNA carriers, targeted agents, diagnostic, imaging, or therapeutic reagents. However, the design of VLP based on IBDV VP2 insertions or substitutions is restricted by the fact that VP2 is a main structural protein of IBDV viral capsid, and insertions or substitutions with particular foreign peptide sequences may result in the inability of the resulting chimeric VP2 protein to self assemble in the form of VLP. In fact, this is clearly exemplified by studies carried out on alternative polyoma VLP [Shin Y C. and Folk W R. (2003) J. of Virology 77(21):11491-11498] were the insertion of peptides often result in VLP disruption.

Therefore, the present invention relates to chimeric fusion proteins of Birnavirus VP2, or fragments thereof that incorporate one or more insertions, or partial substitutions, with particular amino acids or peptides of interest, and which are capable of assembling into VLP structures. More so, the invention relates to methods for the identification and selection of preferred insertion sites within VP2 for the incorporation of peptides of interest without loss of VLP structure and with efficient VLP formation.

DETAILED DESCRIPTION OF THE INVENTION

Virus Like Particles (VLP) are of interest in the design of medicines, therapeutic and prophylactic vaccines as well as in the design of drug delivery systems, carriers for nucleic acids in gene therapy, as targeted agents, imaging agents, in the development of antitoxins and as diagnostic reagents applicable to human and veterinary health. The present invention relates to chimeric VLP of chimeric VP2 fusion proteins, incorporating insertions and/or substitutions with one or more particular amino acids or peptides of interest and methods for the identification and selection of said chimeric VLP.

“Peptides of interest” are hereby defined as amino acid sequences other than IBDV sequences, including vaccine components, antigens and epitopes, targeting sequences, binding sequences, catalytic domains, pharmacology modulators, immunostimulators, toxins and antitoxins which are relevant to human or veterinary health. “Peptides of interest” also include “amino acids of interest” which may be useful in the design of said agents, including those such as lysine, cysteine, tyrosine, histidine, glutamic acid and/or aspartic acid residues that facilitate conjugation of biological entities such as peptides, proteins, DNA, RNA, carbohydrates and small chemical entities relevant to human or veterinary health.

“DNA of interest” refers to DNA sequences encoding for peptides of interest and amino acids of interest.

“VP2” refers to Infectious Bursal Disease Virus (IBDV) VP2 sequences and proteins, including the 512 amino acid VP2 precursor protein (pVP2), the mature 441 VP2 protein, or a fragment of at least 400 amino acids thereof, capable of forming VLP. VP2 includes any VP2 protein found in different IBDV strains, with special reference to those with a protein sequence homology of at least 80% between them. VP2 protein also refers to other Birnaviridae VP2 proteins with protein sequence homologies over 30%, preferably over 40%, and more preferably over 60% with those of IBDV.

“P loop regions” refer to the four loops of the IBDV VP2 P domain named BC (Q₂₁₉-G₂₂₄), DE (R₂₄₉-G₂₅₄), FG (T₂₈₃-D₂₈₇) and HI (S₃₁₅-Q₃₂₄). In brackets the first and last amino acids and corresponding position within VP2 sequence. All other locations within VP2 excluding the C- and N-terminal amino acid are referred to as “Outside P loop regions”.

“Chimeric VP2 fusion proteins” refer to chimeric VP2 proteins incorporating one or more insertions and/or substitutions, at locations other than the C- and N-terminus, with one or more particular amino acids or peptides of interest other than IBDV sequences.

“DNA vectors” refer to DNA sequences that facilitate cloning and expression of VP2 incorporating DNA of interest at the desired insertion or substitution sites. DNA constructs also incorporate “DNA expression vectors” that when expressed in an appropriate host such as bacteria, yeast, insect cells, plants, or mammalian cells, result in VP2, VP2 fusion proteins incorporating peptides of interest, and other IBDV proteins. In the description of said DNA vectors, the insert or substitution is defined in brackets ( ) with arrows ↑ representing the incorporation point of particular peptide of interest X and flanking amino acid positions within the VP2 sequence [e.g.: pESC-URA/VP2(Q₂₁₉↑X↑Y₂₂₀)]. Deletions are represented with a triangle followed by flanking elements of the deleted VP2 peptide sequence [e.g.: pESC-URA/VP2/ΔY₂₂₀-G₂₂₃]. In the definition of multiple lysine (K) substitutions each of the substituted amino acid positions is followed by a K and separated by a hyphen—[e.g.: pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K)].

“Chimeric VP2 VLP” refer to T=1 and T=13 VLP and other nanostructures resulting from the assembly of chimeric VP2 fusion proteins and optionally incorporating IBDV VP3 proteins or fusion proteins thereof.

“VP2-VLP antibodies” refer to anti-VP2 antibodies that are specific for VP2 and VP2 fusion proteins assembled as VLP.

“VLP formation” is determined upon expression of DNA constructs coding for VP2 fusion proteins in the appropriate expression system and quantification by means of a VP2-VLP enzyme linked immunoassay “VLP-ELISA” which makes use of anti-VP2 antibodies capable of recognising VP2 only when assembled as VLP. “VLP formation efficiency” is calculated as a percentage of VLP formation in comparison with that of the native 452 amino acid VP2. Generally, VLP-ELISA values below 20% are regarded as background (BG), and associated chimeric VP2 fusion proteins are considered as not resulting in efficient VLP formation. While VLP formation efficiency values of 20% or higher are regarded as being compatible with VLP formation, that is sufficient for efficient VLP formation, and insertion sites of associated chimeric VP2 fusion proteins can be considered as preferred insertion sites, VLP formation efficiencies above 50% and preferably above 70% are recognised as a desirable feature.

“Preferred insertion and/or substitution sites” refer to locations within VP2 at which the incorporation of amino acids or peptides of interest result in higher VLP formation efficiencies.

IBDV VP2 protein is naturally folded into a helical base (B) domain, a shell (S) domain, and a projection (P) domain and can be made to spontaneously assemble into trimer subunits to form icosahedral VLP T=1, or T=13 in the presence of VP3. In the present invention it has been observed that the insertion of a peptide of interest within IBDV VP2 often results in destabilisation of VLP structure. Furthermore it has been observed that for many possible insertion or substitution sites, VLP formation efficiency of the resulting chimeric VP2 fusion proteins depends on the sequence of the particular peptide of interest inserted. The present invention is directed towards the identification of preferred sites within VP2 which are appropriate for the insertion or substitution with peptides of interest while maintaining VLP structure, methods for the identification of said preferred insertion or substitution sites, and the resulting chimeric VP2 VLP.

Furthermore, the present invention is directed towards chimeric VP2 VLP incorporating one or more insertions or substitutions with particular amino acids and peptides of interest within the four VP2 P loops, BC, DE, FG and HI and/or locations outside said P loops.

Most of the particular embodiments of the present invention, have been exemplified (examples 1 to 15) with five different example peptides selected for the purpose of this invention, namely TS (SEQ. ID. NO: 1), Flag (SEQ ID NO 2), cMyc (Seq SEQ ID NO 3), V5 (SEQ ID NO 4) and VSV-G (SEQ ID NO 5).

Chimeric VP2 Fusion Proteins Incorporating Insertions and/or Substitutions Within P Loop Regions.

The BC, DE, FG and HI loops of the IBDV VP2 P domain represent possible insertion sites for fusion proteins incorporating particular amino acids and peptides of interest. This is explored in the present invention through the incorporation of five different example peptides of interest, namely TS (SEQ. ID. NO: 1), Flag (SEQ. ID. NO: 2), cMyc (SEQ. ID. NO: 3), V5 (SEQ. ID. NO: 4) and VSV-G (SEQ. ID. NO: 5), at all the possible insertion points within VP2 P loops as shown in Example 1.

Incorporation of the Threonine-Serine (SEQ. ID. NO: 1) sequence coding for a SpeI restriction site at all possible insertion sites within the BC, DE, FG and HI P loop regions and closely adjacent positions resulted in the identification of preferred insertion points. Many insertion points within the P loop regions appeared to be compatible with VLP formation. As depicted in FIG. 2, incorporation of the SpeI restriction site within the TS sequence facilitates subsequent incorporation of DNA sequences encoding for the other example peptides of interest, namely Flag (SEQ. ID. NO: 2), cMyc (SEQ. ID. NO: 3), V5 (SEQ. ID NO: 4) and VSV-G (SEQ. ID. NO: 5). Insertion of these different model peptides of interest, at each of the possible VP2 P loop region positions, identified suitable insertion sites and demonstrated that preferred sites within the BC, DE, FG and HI P loops varied according to the inserted DNA of interest. Generally the incorporation of peptides of interest in VP2 regions adjacent to the BC, DE, FG and HI P loops does not result in significant VLP formation efficiency and thus does not appear to be compatible with VLP formation.

Furthermore, as demonstrated in Example 2, VP2 of different lengths, exemplified by IBDV VP2 of different lengths at their C-terminal, namely VP2 with 452, 441 and 456 amino acids (VP2 452, VP2 441 and VP2 456), also permit the insertion of the example peptides of interest, namely Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3), into P loop regions with little variation of VLP formation efficiency. Generally insertion intoVP2 456 resulted in a reduction of VLP formation efficiency compared to VP2 452 or VP2 441 with the same insertion. VP2 452 and VP2 441 both represent preferred lengths for the formation of VLP incorporating peptides of interest. It is envisaged that VP2 proteins of at least 400 amino acids, other than 452, 441 and 456, may also be capable of effectively forming VLP and chimeric VP2 VLP.

Therefore, the present invention incorporates chimeric VP2 fusion proteins and resulting chimeric VP2 VLP where amino acids or peptides of interest are inserted at the BC (Q₂₁₉-G₂₂₄), DE (R₂₄₉-G₂₅₄), FG (T₂₈₃-D₂₈₇) and HI (S₃₁₅-Q₃₂₄) P loop regions of VP2 protein and fragments thereof. The present invention also incorporates the DNA vectors and constructs that permit cloning DNA of interest at each of the available sites within the VP2 P loop regions and DNA expression vectors for the expression of the resulting chimeric VP2 P loop region fusion proteins.

IBDV VP2 P loop regions not only represent potential insertion sites for particular amino acids or peptides of interest but also represent possible sites for the substitution of VP2 amino acids for peptides of interest. Locations and/or structural elements of VP2 which have been shown to be compatible with an insertion may also be considered as potential sites for substitutions with peptides of interest. Substitution of one or more amino acid residues adjacent, or close, to the insertion site may be explored or alternatively entire structural elements, such as a connecting loop, or parts of it may be substituted. VP2 P loop region substitutions are exemplified in Example 3 where amino acids within the BC, DE, FG and HI VP2 P loop regions are substituted by example peptides of interest, namely Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3), and resulting constructs evaluated for their capacity to form VP2 VLP. Removal of the P loop regions while maintaining the first and last residue of the P loop regions appeared to be compatible with VLP formation. Furthermore, although with a lesser, but still equal or above 20% efficiency, for two of the expression vectors and inserted DNA of interest, the substitution of an entire P loop region also resulted in VLP formation depending on the site of substitution. Therefore, incorporation of peptides of interest within VP2 can be directed towards the substitution of the entire P loop regions, or preferably only parts of them. Furthermore, the peptides of interest introduced as substitutions in the BC, DE, FG and HI VP2 P loop regions may be of same or different length than the P loop regions for which they have been substituted.

The present invention therefore incorporates chimeric VP2 fusion proteins and chimeric VP2 VLP resulting from the substitution of VP2 P loop regions, or fragments thereof, by particular peptides and amino acids of interest at one or more locations within the P loop regions. The present invention also incorporates the DNA vectors and constructs that permit cloning DNA of interest at each of the available substitution positions within the VP2 P loop regions, and DNA expression vectors for the expression of the resulting chimeric VP2 P loop fusion proteins.

Another realisation of VLP resulting from VP2 fusion proteins is exemplified in Example 4 which describes the substitution with lysine (K) residues at different points within BC, DE, FG and HI VP2 P loop regions and the chemical conjugation of the example peptide cMyc (SEQ. ID. NO: 3). Incorporation of K residues in VP2 P loop regions permits the chemical conjugation of the resulting VLP with multiple copies of biological and chemical entities containing, or made to contain, cysteine residues at the desired conjugation points. The incorporation of K residues may involve the insertion or substitution of VP2 amino acid residues by K, poly K, or K rich peptides, at the VP2 P loop regions or outside said P loops. Furthermore, desired conjugation points within VP2 may be substituted, or made to contain, amino acid sequences, other than K residues, that facilitate chemical conjugation with biological and chemical entities by other means of conjugation or coupling such as, but not limited to, cysteines, tyrosine, histidine, glutamic acid, or aspartic acid. Preferred substitution and/or insertion sites for K residues or other residues that permit conjugation include those found to favour higher VLP formation efficiency.

The present invention therefore incorporates chimeric VP2 VLP resulting from the insertion or substitution of VP2 amino acid residues by residues that facilitate chemical conjugation of biological and chemical entities that may contribute towards the desired biological or pharmacological properties of the chimeric VLP. The present invention also incorporates DNA vectors and DNA expression vectors for the expression of chimeric VP2 VLP incorporating amino acid residues that facilitate chemical conjugation.

Chimeric VP2 Fusion Proteins Incorporating Insertions and/or Substitutions Outside P Loop Regions.

Potential insertion of foreign peptides in the B, S or P domains of VP2 outside the BC (AA 219-224), DE (AA 249-254), FG (AA 283-287) and HI (AA 315-324) P loop regions, most likely result in disruption of VLP structure but may also represent a means for modulating surface exposure of the inserted peptides of interest or their biological activity. In the present invention regions outside the VP2 P loop regions which permit the incorporation of particular peptides of interest by insertion or substitution without loss of VLP structure are identified by means of transponson insertion mutagenesis using transposons Tn5 and Mu.

As shown in Example 5, a random screen with Tn5 generated a DNA library of clones with Tn5 insertions along the complete VP2 452 amino acid sequence. Evaluation of the capacity of these constructs to produce VLP resulted in the identification of a number of insertion sites that result in VP2 insertion fusion proteins that retain VLP structure. Similarly, as shown in Example 6, a random screen with transposon Mu generated a DNA library of clones with insertions along the complete VP2 452 amino acid sequence. Evaluation of the capacity of these constructs to produce viable VLP lead to the identification of a number of potential insertion sites that result in VP2 insertion fusion proteins that retain VLP structure.

As exemplified in Examples 5 and 6, insertion locations identified with Tn5 and Mu transposon mutagenesis vary depending on the transposon sequences used. Other transposons or means for random insertion mutagenesis may also be used for the identification of additional sites within VP2 with potential for the insertion and/or substitution with peptides of interest. Transposon insertion mutagenesis permits the identification of locations within the VP2, or chimeric VP2 fusion proteins, which can accommodate insertions and or substitutions while maintaining VLP forming capacity. It is envisaged that by exhaustive evaluation of random insertion libraries, and by means of using different transposons, all possible VP2 insertion sites compatible with VLP formation can be identified. As previously described, those identified VP2 insertion sites may also be considered as potential sites for substitutions with peptides of interest. Identified transposon insertion sites represent potential insertion sites for DNA of interest. As shown in Example 7, insertion of said DNA of interest may be carried out into the inserted transposon sequences, or through the substitution of the inserted transposon sequences. In said Example 7, the presence of a restriction enzyme site such as NotI within the inserted Tn5 and Mu transposons facilitates the insertion of example DNA of interest, namely Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3), into the inserted transposon sequences. However, this results in inserts which may contain undesirable sequences derived from the originally inserted transposon. Alternatively, the identification of the transposon insertion site permits the introduction of a unique cloning site, such as a SpeI, at the identified location in VP2. As shown in Example 7, the unique cloning site permits the insertion of DNA of interest and generally results in increased VLP formation efficiencies compared to chimeric VP2 fusion proteins which conserve transposon derived sequences. Therefore in a preferred embodiment of the present invention a unique cloning site is engineered at locations within VP2, or chimeric VP2 fusion proteins, originally identified by transposon mutagenesis or other means of random insertion of DNA of interest. Resulting VP2 insertion vectors can then be used for the incorporation of DNA of interest at the desired insertion point for their expression in appropriate expression systems and evaluation of VLP forming capacity. If desired, chimeric VP2 fusion proteins showing the best formation efficiencies or desired properties can be further optimised through the removal of the TS sequences resulting from the engineered unique cloning site by standard genetic engineering techniques. It is acknowledged that the incorporation of DNA of interest can also result in the total or partial substitution of VP2 amino acids closely adjacent to the identified location site.

Therefore, the present invention incorporates chimeric VP2 VLP resulting from the assembly of chimeric VP2 fusion proteins which incorporate insertions and/or substitutions with particular amino acids or peptides of interest at locations outside the VP2 P loop regions. Furthermore, the present invention also incorporates VP2 DNA vectors and DNA expression vectors incorporating one or more insertions and/or substitutions with DNA of interest at locations outside the P loop regions.

Chimeric VP2 Fusion Proteins Incorporating Multiple Insertions and/or Substitutions.

The identification of preferred insertion and/or substitution sites provides the means for insertion or substitution with peptides of interest at more than one site within VP2. This is exemplified in Example 4 were Lysine (K) residues have been inserted at 1, 2, 3 or 4 locations simultaneously while maintaining the overall VLP structure. Substitution with multiple Lysine residues facilitates the conjugation with multiple copies of a biological or chemical entity, such as nucleic acids, peptides, carbohydrates and small molecules, which may be a desirable feature for purification, targeting, drug loading or in altering VLP surface chemistry for improved pharmacology. Another example of chimeric VP2 VLP resulting from the incorporation of peptides of interest at more than one point within VP2 is provided by Example 8 in which example peptides of interest, cMyc (SEQ. ID. NO: 3) and Flag (SEQ. ID. NO: 2), are inserted and or substituted at more than one location within VP2. Introduction of peptides of interest can be carried out by the introduction of cloning sites at the desired insertion and/or substitution points, by cloning insertion containing fragments, or following other standard molecular biology procedures. Therefore the present invention incorporates chimeric VP2 fusion proteins, and resulting chimeric VP2 VLP, incorporating more than one insertion and/or substitution and DNA expression vectors for expression of said chimeric VP2 fusion proteins.

Chimeric VP2 Fusion Proteins Incorporating Insertions and/or Substitutions and Terminal Fusions.

Furthermore, another object of the present invention incorporates VLP in which chimeric VP2 insertion or substitution fusion proteins are in addition also fused, either at their carboxy (C-) or amino (N-) terminal end, to a peptide of interest which may be the same or different to the inserted peptides of interest. Chimeric VP2 insertion or substitution fusion proteins with additional terminal fusions are exemplified in Example 9 were example peptides of interest, Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3), are inserted at various points within VP2 P loop regions and in addition also fused at the VP2 C- or N-terminus. VLP formation efficiency was found to depend on both, the location of the primary insertion, and the inserted peptide. Furthermore, incorporation of the additional terminal fusion generally results in decreased, but still equal or above 20%, VLP formation efficiencies compared to single insertion chimeric VP2 fusion proteins. As shown in Example 9, for the particular peptides of interest evaluated, some constructs incorporating both, the insertion and terminal fusion, appeared to be compatible with VLP formation.

The present invention therefore incorporates chimeric VP2 VLP resulting from VP2 insertion or substitution fusions with peptides of interest and additionally, the terminal fusion of the resulting insertion or substitution with same or different peptides of interest. Identification of the preferred DNA constructs incorporating insertions and terminal fusions may be carried out following the initial identification of the preferred insertion points followed by a C- or N-terminal fusion, or alternatively the terminally fused chimeric VP2 protein may be screened or evaluated for preferred insertion or substitution points. This screen may be carried out against pre-selected sites within VP2, or randomly following transposon mutagenesis or other random cloning approaches.

The present invention also incorporates the DNA vectors and constructs that permit cloning DNA of interest at each of the available insertion or substitution positions within a VP2 protein that is also terminally fused, either at their carboxy (C-) or amino (N-) terminal end, to a peptide of interest which may be the same, or different, to the inserted or fused amino acids or peptides of interest.

Chimeric VLP Containing Chimeric VP2 Fusion Proteins and Other Birnavirus Derived Proteins.

Furthermore, as exemplified in Examples 10 to 12, VP2 fusion protein incorporating the insertion or substitution of one or more amino acid or particular peptides of interest may also be expressed simultaneous with other IBDV or Birnaviridae proteins to favour the formation of T=13 VLP. Formation of T=13 VLP, compared to T=1 VLP, increases the copy number of VP2 proteins per VLP and may result in improved VLP stability and/or may be a preferred form for presentation or incorporation of peptides of interest. Recombinant T=13 VLP can either be generated by the expression of the pVP2-VP4-VP3 polyproteins, or by co-expression of the pVP2 and VP3 gene.

The expression of pVP2-VP4-VP3 polyprotein results in the generation of the individual proteins pVP2, VP4 and VP3 through the proteolytic activity of VP4. Expression of wild-type IBDV or Birnaviridae polyproteins usually gives rise to tubular structures containing pVP2. However, fusion of an exogenous sequence at the C-terminus of VP3, such as GFP, or the deletion of C-terminal VP3 residues strongly promote pVP2 processing and the self assembly of T=13 VLP. As exemplified in Example 10, T=13 VLP incorporating a chimeric VP2 insertion fusion protein pVP2(X)-VP4-VP3-Y containing the example peptide of interest (X or Y), cMyc (SEQ. ID. NO: 3) or Flag (SEQ. ID. NO: 2), and also VP3-Flag or VP3-cMyc, may be efficiently formed by the expression in baculovirus expression systems. Furthermore the VP3 component of the polyprotein can also be made to contain peptides of interest as exemplified by the terminal fusion of example peptides of interest, cMyc (SEQ. ID. NO: 3) and Flag (SEQ. ID. NO: 2).

Alternatively, co-expression of pVP2 and VP3 from independent gene constructs also provide the means for the formation of T=13 VLP. Furthermore, the expressions of pVP2 and VP3 fused at its N-terminus to several Histidine residues result in T=13 VLP which, in contrast to polyprotein expression systems, can be made to contains unprocessed pVP2. As shown in Example 11, co-expression in yeast of chimeric VP2 fusion proteins incorporating Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3) as example insertions at P loop regions, and His-VP3 incorporating Flag and cMyc as an example terminal fusion, results in efficient formation of T=13 VLP. Furthermore, the incorporation of multiple lysine (K) residues within the VP2 P loop regions also results in acceptable T=13 VLP formation. Incorporation of said K residues is aimed at the chemical conjugation to T=13 VLP of multiple copies of biological and chemical entities in a similar fashion as that described herein for T=1 VLP. Similarly, as shown in Example 12, T=13 VLP incorporating a chimeric VP2 fusion protein containing as insertion an example peptide of interest cMyc (SEQ. ID. NO: 3) or Flag (SEQ. ID. NO: 2), and His-VP3 which is C-terminally fused to same or different peptide of interest, may also be efficiently formed by the expression in baculovirus expression systems.

Therefore, another object of the present invention refers to T=13 VLP that result from the assembly of chimeric VP2 fusion proteins, resulting from the insertion or substitution with one or more amino acids or particular peptides of interest within VP2, and VP3 proteins that may, or may not, incorporate the same or other peptides of interest. The present invention also incorporates DNA expression vectors and constructs incorporating pVP2(X)-VP4-VP3-Y polyprotein, where X an Y represent a particular DNA of interest, or alternatively DNA expression vectors that permit the simultaneous expression of chimeric VP2 fusion proteins incorporating peptides of interest and VP3 proteins or fusion proteins thereof.

Screening of Pre-Selected VP2 DNA Vectors for Preferred Insertion and/or Substitution Sites for Particular Peptides of Interest.

Preferred insertion or substitution sites within VP2, judged by the ability to form VLP efficiently, may vary for different peptides of interest. Therefore, another object of the present invention demonstrated in Examples 13 and 14 incorporates screening methods to identify preferred insertion sites for given peptides of interest using a pre-selected panel of VP2 insertion and/or substitution DNA vectors. The screening methods can be generally conducted as follows:

-   -   1. Selection of VP2 vectors: Whereby a selection panel of VP2         DNA insertion and/or substitution vectors is made to incorporate         a cloning site, preferably a multiple cloning site, which         favours directional cloning of the DNA of interest.     -   2. Cloning of DNA of interest: Whereby the DNA of interest is         cloned following standard molecular biology procedures into the         previously selected panel of VP2 insertion and/or substitution         vectors. DNA vectors may be arranged as an array of pre-selected         vectors, one vector per well as shown in Example 13, or in a         pool format containing more than one pre-selected vector per         cloning reaction as shown in Example 14. The latter resulting in         VP2 insertion and/or substitution libraries.     -   3. Transformation and expression: The resulting ligations or VP2         insertion and/or substitution libraries obtained in the previous         step are transformed into suitable, bacterial, yeast, or other         suitable expression systems for the evaluation of VLP formation         efficiency of the individual clones. This may be carried out in         individual wells for array formats, or may be carried out as a         pool followed by plating and colony selection.     -   4. Evaluation of VLP formation capacity: For individual clones         resulting from the array screen, host cells are lysed and         analysed for VLP expression in each of the individual wells or         spots by means of a VLP-ELISA. For pooled transformations,         resulting plated colonies can be assessed for VLP production         efficiency by means of a colony immunoblot assay using VP2 VLP         specific antibodies against replica filters made to contain         samples for each of the colonies. The chimeric VP2 fusion         proteins with highest VLP formation efficiencies are selected         for the confirmation and identification of the preferred         insertion site. While VLP formation efficiencies above 20% are         recognised as compatible with VLP formation, VLP formation         efficiencies above 50%, and more preferably above 70%, are         recognised as a desirable feature for resulting chimeric VP2         fusion proteins.     -   5. Confirmation or identification of insertion site: DNA         sequencing techniques provide the means for confirmation or, for         pooled transformations, the identification of the site of         insertion of the DNA of interest. Furthermore VLP formation can         be confirmed by electron microscopy (EM).

These screening methods against pre-selected VP2 DNA vectors permit the rapid selection of the best pre-selected sites within VP2 for the incorporation, either as insertions or substitutions, of a particular peptide of interest resulting in VLP production. These screening methods can be carried out either in an array format or in a pool format to generate libraries that can then be screened for colonies that are expressing chimeric VP2 fusion proteins that result in efficient VLP formation.

The present invention therefore incorporates screening methods that involve array arrangements of VP2 DNA vectors for the insertion at pre-selected points of a particular DNA of interest, and the VP2 fusion proteins resulting from the expression of the resulting chimeric VP2 fusion DNA expression vectors. Furthermore, the present invention also incorporates screening methods that involve the use of VP2 DNA vector pools for the insertion of DNA of interest at pre-selected points, and subsequent selection of chimeric VP2 fusion protein expression vectors which result in efficient VLP formation. Said screening methods either in an array format or in a pool format, can be carried out not only against VP2 452 but also against VP2 of different lengths or previously made to contain insertions or substitutions with the same or different peptides of interest.

Random Screening for Preferred Insertion and/or Substitution Sites for Particular Peptides of Interest.

Preferred insertion or substitution sites within VP2, judged by the ability to form VLP efficiently, may vary for different peptides of interest. Therefore, another object of the present invention demonstrated in Example 15 incorporates random screening methods to identify preferred insertion sites for a given peptide of interest using a random VP2 insertion and/or substitution library for the incorporation of peptides of interest. The random screening methods can be generally conducted as follows:

-   -   1. Generation of random insertions: Whereby any transposon         containing appropriate cloning site is made to randomly insert         throughout the entire VP2 gene to produce a transposon insertion         library. The incorporation of one, or preferably two, unique         restriction enzyme sites within the randomly inserted transposon         derived sequences facilitates directional cloning of the desired         DNA of interest.     -   2. Cloning of DNA of interest: Whereby the DNA of interest is         cloned into the previously generated transposon insertion         library following standard molecular biology procedures.     -   3. Transformation and expression: The resulting ligation library         is then transformed into suitable, bacterial, yeast, or other         suitable expression systems for the evaluation of VLP formation         efficiency of the individual colonies.     -   4. Evaluation of VLP formation capacity: Colonies can be         assessed for VLP production efficiency by means of a colony         immunoblot assay using VP2 VLP specific antibodies against         replica filters made to contain samples for each of the         colonies. The chimeric VP2 fusion proteins with highest VLP         formation efficiencies are selected for the confirmation and         identification of the preferred insertion site. While VLP         formation efficiencies above 20% are recognised as compatible         with VLP formation, VLP formation efficiencies above 50%, and         more preferably above 70%, are recognised as a desirable feature         for resulting chimeric VP2 fusion proteins.     -   5. Confirmation or identification of insertion site: DNA         sequencing techniques provide the means for identification of         the site of insertion of the DNA of interest. Furthermore VLP         formation can be confirmed by electron microscopy (EM).

These random screening methods permit the rapid selection of the best pre-selected sites within VP2 for the incorporation of a particular peptide of interest resulting in VLP production. The present invention therefore incorporates random screening methods for the identification of preferred VP2 insertion sites for a particular DNA of interest, and the chimeric VP2 fusion proteins, and chimeric VP2 VLP, resulting from the expression of the resulting chimeric VP2 fusion protein DNA expression vectors. Furthermore, said screening methods can be carried out not only against VP2 452 but also against VP2 of different lengths or already made to contain insertions or substitutions with peptides of interest.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made there to without departing from the spirit or scope of the appended claims.

DESCRIPTION OF THE FIGURES

FIG. 1: Oligonucleotides Used for Plasmid Constructions.

FIG. 1 describes the nucleotide sequences used as primers for PCR reactions in the construction of DNA vectors and DNA expression vectors.

FIG. 2: Construction of SpeI-VP2 Loop Insertion Vectors.

FIG. 2 depicts the insertion of SpeI restriction sites by site directed mutagenesis generating Serine-Threonine (TS) inserts at each of the possible positions within the VP2 P loop DE region. Cloning of DNA of interest into the SpeI sites led the insertion of peptides of interest flanked by TS dipeptides on both ends.

FIG. 3: Electron Microscopy Analysis of Purified Chimeric VP2 VLP.

FIG. 3 depicts electron micrographs (EM) of chimeric VLP obtained following DNA expression vector expression in S. cerevisiae strain 499, lysis and fractionation of soluble extracts in sucrose gradients. Panels a-i show electron microscopy (EM) pictures of purified chimeric VP2 VLP stained with uranyl actetae: (a) VP2(H₂₅₃↑Flag↑G₂₅₄), (b) VP2(H₂₅₃↑cMyc|G₂₅₄), (c) VP2(A₃₂₁↑Flag↑G₃₂₂), (d) VP2(A₃₂₁↑cMYc↑G₃₂₂), (e) VP2(V₂₅₂↑Flag↑H₂₅₃), (f) VP2(H₂₅₃↑Flag↑G₂₅₄)-Flag, (g) VP2(H₂₅₃↑cMyc↑G₂₅₄)-cMyc, (h) VP2(A₃₂₁↑Flag↑G₃₂₂)-Flag and (i) VP2(Q₂₂₁↑K↑-H₂₅₃↑K↑-G₂₈₅↑K↑H₃₂₀↑K↑). The bar shown in the EM pictures corresponds to 200 nm.

FIG. 4: Construction of VP2 Expression Plasmids with Multiple Insertions.

FIG. 4 depicts VP2 expression constructs with two Flag or cMyc insertions in P loop DE and HI regions pESC-URA/NP2(X-X). Double insertions result from cloning RsrII and NarI fragments of pESC-URA/VP2(H₂₅₃↑cMyc↑G₂₅₄) and pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄) into pESC-URA/VP2(A₃₂₁↑cMyc↑G₃₂₂) and pESC-URA/VP2(A₂₅₃␣Flag␣G₃₂₂).

FIG. 5: Construction of VP2 Insertion and Substitution Constructs with N- or C-Terminal Fusion.

FIG. 5 depicts the C-terminal and N-terminal fusion of example peptides of interest cMyc and Flag to chimeric VP2 fusion proteins previously made to contain cMyc, Flag or multiple Lysine substitutions. C-terminal constructs pESC-URA/VP2(X)-X were generated by insertion of Flag and cMyc peptide encoding DNA adapters downstream of the VP2 genes using restriction sites NotI and HindIII. N-terminal constructs pESC-URA/X-VP2(X) were generated by insertion at an EcoRI site located upstream of the VP2 start codon.

FIG. 6: Sequence of Synthetic pVP2 Gene.

FIG. 6 show the nucleotide sequence of the synthetically produced pVP2 gene of IBDV Soroa strain (NCIB No. AAD30136) and the corresponding protein. DNA restriction sites used for cloning are underlined and named. Amino acid residues of P loop regions BC, DE, FG and HI are shown underlined.

FIG. 7: Sequence of Synthetic VP4 Gene.

FIG. 7 shows the nucleotide sequence of the synthetically produced fragment of the IBDV Soroa strain segment A (NCIB No. AAD30136), which contains the 3′end of the pVP2 gene, the VP4 gene and the 5′ end of the VP3 gene. The VP4 amino acid sequence is numbered and underlined and DNA restriction sites used in cloning are underlined and named.

FIG. 8: Sequence of Synthetic VP3 Gene.

FIG. 8 shows the nucleotide sequence of the synthetically produced VP3 gene of IBDV Soroa strain (NCIB No. AAD30136) and the corresponding protein. Silent mutations have been introduced at positions 147 (A to T), 333 (C to T), 561 (T to C) and 600 (T to C) to mutate restriction enzyme sites Mscl, NcoI, HindIII and NcoI, respectively.

FIG. 9: Construction of pFastBacDual pVP2-VP4-VP3-pp Expression Vectors.

FIG. 9 shows the steps for the construction of pFastBacDual pVP2-VP4-VP3-pp expression vectors. Step 1: Cloning of pVP2 gene into pFastBacDual™ (pFBD) downstream of PH promoter; Step 2: insertion VP3-X genes, Flag and cMyc, at HindIII restriction site downstream of pVP2 gene; Step 3: Substitution of pVP2 RsrII-NcoI fragment to generate pVP2 insertion or substitution constructs pFBD/pVP2(X)-VP3-X-pp containing Flag, c Myc or multiple lysines; Step 4: Insertion of VP4 gene at restriction sites PvuII and BsgI located at the 3′ and 5′ end of the pVP2 and VP3 gene, respectively, to generate pFBD/pVP2(X)-VP4-VP3-X-pp.

FIG. 10: Construction of Dual pESC-URA pVP2, VP3 Expression Vectors:

FIG. 10 shows the steps for the construction of dual pESC-URA pVP2, VP3 expression vectors. Step 1: Cloning of His-VP3-X into pESC-URA™ downstream of G10 promoter to generate pESC-URA/His-VP3-X; Step 2: Insertion of pVP2 genes downstream of G1 promoter at a BamI II-HindIII site, generating dual pVP2-VP3 expression vectors pESC-URA/pVP2-His-VP3-X; Step 3: Substitution of pVP2 RsrII-MscI fragment to generate pVP2-VP3 insertion or substitution constructs pESC-URA/pVP2(X)-His-VP3-X.

FIG. 11: Construction of FastBacDual pVP2, VP3 Expression Vectors.

FIG. 11 shows the steps for the construction of FastBacDual pVP2, VP3 expression vectors. Step 1: Cloning of His-VP3-X genes into pFastBacDual™ (pFBD) downstream of P10 promoter to generate pFBD/His-VP3-X, Step 2: Insertion of pVP2 genes downstream of PH promoter at BamHI-HindIII site, generating dual pVP2-VP3 expression vectors pFBD/pVP2-His-VP3-X; Step 3: Substitution of pVP2 RsrII-NcoI fragment to generate pVP2-VP3 insertion or substitution constructs pFBD/pVP2(X)-His-VP3-X.

FIG. 12: Electron Microscopy (EM) Analysis of Purified Chimeric T=13 VLP.

FIG. 12 shows electron microscopy (EM) pictures of purified T=13 VLP samples stained with uranyl actetae following sucrose gradient purification. (1) pVP2(H₂₅₃↑Flag↑G₂₅₄)-His-VP3-Flag and (2) pVP2-His-VP3-Flag. The bar shown in the EM pictures corresponds to 200 nm.

FIG. 13: Generation of a Multiple Cloning Site at VP2 Insertion Positions.

FIG. 13 depicts the insertion of a multiple cloning site (MCS) at a SpeI site resulting in a NotI/SpeI cloning site containing several in-frame stop codons. Peptides of interest are introduced as NotI/SpeI insertions. Stop codons ensure that re-ligation of empty vectors during the generating of insertion libraries generate deleted versions of the VP2 protein that are unable to form VLP.

EXAMPLES Example 1 Insertion of Peptides of Interest in P Loop Regions of IBDV VP2

A collection of plasmids incorporating a SpeI site at each of the possible positions within the VP2 P loops, and immediately adjacent positions, was generated by site-directed mutagenesis using the yeast expression plasmid pESC-URA-VP2 452 (pESC-URA/VP2). For the construction of pESC-URA/VP2 the VP2 cDNA was amplified using oligonucleotides VP2-452EcoRI-fw (SEQ. ID. NO: 6) and VP2-452NotI-rev (SEQ. ID. NO: 7) (FIG. 1) and templates pESC-URA/pVP2-512 which contains full-length IBDV VP2 512 insert (IBDV Soroa strain, NCIB No. AAD30136). The VP2 452 I nsert was coned into EcoRI and NotI digested pESC-URA (Stratagene™) in which SpeI site was previously deleted. The insertion of the SpeI restriction site generated a Serine-Threonine (TS) insertion at each of the possible positions within the VP2 P loop regions as shown in Table 1 and exemplified for P loop region DE in FIG. 2. Purified SpeI-VP2 loop insertion vectors (VP2-TS loop insertion vectors or pESC-URA/VP2/Spe) were separately linearised by digestion with SpeI and ligated to DNA adapter molecules encoding for a single copy of the peptide of interest. Re-ligated plasmids were used to transform E. coli, and insertion clones were identified by restriction analysis and sequencing. This was carried out for each of 4 example peptides of interest, namely Flag (SEQ. ID. NO: 2), cMyc (SEQ. ID. NO: 3), V5 (SEQ. ID. NO: 4) and VSV G (SEQ. ID. NO: 5) generating a panel of VP2-peptide insertion constructs with flanking TS sequences derived from the engineered SpeI site. Purified VP2-TS loop insertion vectors and VP2-peptide insertion constructs were used to transform S. cerevisiae Y449 strain. The ability to form VLP for each of the VP2-TS loop insertion vectors [e.g.: pESC-URA/VP2(H₂₅₃↑S↑G₂₅₄)] and VP2-peptide loop insertion constructs [e.g.: pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄)] was quantitatively determined by VLP-ELISA using total yeast cell extracts (see Table 1). The VLP-ELISA assay made use of an antibody capable of recognising VP2 only when assembled in the form of a VLP. Briefly, ELISA plates were pre-coated with rabbit anti-VP2 and serial sample dilutions were added and incubated for 1 hour at RT. After washing, the plate is incubated with mouse anti-VP2 for 1 hour at RT followed by peroxidase assay development according to standard procedure. The EC50 for a given sample is determined as the sample dilution at which the VLP-ELISA results in 50% of the maximum signal obtained for that sample. VLP formation efficiency is in all cases expressed as the percentage of the EC50 value obtained for a given sample compared to the native VP2 452 control.

In order to confirm chimeric VP2 VLP formation, large scale yeast cultures were prepared, VLP were purified by means of sucrose gradient centrifugation, and the presence of VLP verified by electron microscopy (EM) as shown in FIG. 3. VLP formation efficiencies below 20% of that of the native VP2 452 were considered as background (BG) and were taken to be an indication that the insertion location was not compatible with VLP formation for the tested peptides. The obtained results served to identify insertion sites specifically favoured by the different tested peptides of interest.

As demonstrated in Table 1 VLP formation efficiency of inserts at the BC, DE, FG and HI loops of the VP2 P domain depends on the sequence of the inserted DNA of interest and the insertion location. As an example, insertion locations, indicated by an arrow, which resulted in high VLP formation efficiency for the TS di-peptide included positions G₂₅₄↑L₂₅₅, D₂₈₇↑N₂₈₈, S₃₁₅↑K₃₁₆, K₃₁₆↑S₃₁₇, G₃₂₂↑D₃₂₃ and D₃₂₃↑Q₃₂₄, while the preferred sites for Flag insertion were H₂₅₃↑G₂₅₄, S₃₁₅↑K₃₁₆ and G₃₁₈↑G₃₁₉, for cMyc insertion D₃₂₃↑Q₃₂₄, for V5 insertion G₃₁₈↑G₃₁₉ and Q₃₂₄↑M₃₂₅, and for VSV-G insertion the G₃₁₈↑G₃₁₉ position.

TABLE 1 Insertion positions in P loop regions BC, DE, FG and HI. P-LOOP INSERT POSITION % VLP REGION (P-loop region sequences underlined) TS Flag cMyc V5 VSV-G B-C ↑S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ V₂₂₅ 22 BG BG BG BG S₂₁₈↑Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ V₂₂₅ 48 35 BG 25 BG S₂₁₈ Q₂₁₉↑Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ V₂₂₅ 23 BG 36 80 84 S₂₁₈ Q₂₁₉Y₂₂₀↑Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ V₂₂₅ 47 34 61 33 82 S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁↑P₂₂₂G₂₂₃G₂₂₄ V₂₂₅ 37 31 BG BG 20 S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂↑G₂₂₃G₂₂₄ V₂₂₅ 65 46 34 71 75 S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃↑G₂₂₄ V₂₂₅ BG BG 30 BG BG S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ ↑V₂₂₅ 34 23 BG BG 25 S₂₁₈ Q₂₁₉Y₂₂₀Q₂₂₁P₂₂₂G₂₂₃G₂₂₄ V₂₂₅↑ BG BG BG BG BG D-E ↑F₂₄₈ R₂₄₉T₂₅₀S₂₅₁V₂₅₂H₂₅₃G₂₅₄ L₂₅₅ BG NT NT NT NT F₂₄₈↑R₂₄₉T₂₅₀S₂₅₁V₂₅₂H₂₅₃G₂₅₄ L₂₅₅ BG NT NT NT NT F₂₄₈ R₂₄₉↑T₂₅₀S₂₅₁V₂₅₂H₂₅₃G₂₅₄ L₂₅₅ BG BG BG BG BG F₂₄₈ R₂₄₉T₂₅₀↑S₂₅₁V₂₅₂H₂₅₃G₂₅₄ L₂₅₅ 43 37 BG BG 23 F₂₄₈ R₂₄₉T₂₅₀S₂₅₁↑V₂₅₂H₂₅₃G₂₅₄ L₂₅₅ 38 BG BG BG BG F₂₄₈ R₂₄₉T₂₅₀S₂₅₁V₂₅₂↑H₂₅₃G₂₅₄ L₂₅₅ 26 66 BG 25 24 F₂₄₈ R₂₄₉T₂₅₀S₂₅₁V₂₅₂H₂₅₃↑G₂₅₄ L₂₅₅ 69 100  84 NT 49 F₂₄₈ R₂₄₉T₂₅₀S₂₅₁V₂₅₂H₂₅₃G₂₅₄ ↑L₂₅₅ 100  28 BG BG BG F₂₄₈ R₂₄₉T₂₅₀S₂₅₁V₂₅₂H₂₅₃G₂₅₄ L₂₅₅↑ 35 25 22 BG NT F-G ↑L₂₈₄ T₂₈₃T₂₈₄G₂₈₅T₂₈₆D₂₈₇ N₂₈₈ 25 BG NT NT NT L₂₈₄↑T₂₈₃T₂₈₄G₂₈₅T₂₈₆D₂₈₇ N₂₈₈ 62 45 34 23 BG L₂₈₄ T₂₈₃↑T₂₈₄G₂₈₅T₂₈₆D₂₈₇ N₂₈₈ 79 45 23 95 80 L₂₈₄ T₂₈₃T₂₈₄↑G₂₈₅T₂₈₆D₂₈₇ N₂₈₈ 81 56 45 68 70 L₂₈₄ T₂₈₃T₂₈₄G₂₈₅↑T₂₈₆D₂₈₇ N₂₈₈ 85 75 NT NT NT L₂₈₄ T₂₈₃T₂₈₄G₂₈₅T₂₈₆↑D₂₈₇ N₂₈₈ 90 66 32 NT 57 L₂₈₄ T₂₈₃T₂₈₄G₂₈₅T₂₈₆D₂₈₇ ↑N₂₈₈ 97 62 NT NT NT L₂₈₄ T₂₈₃T₂₈₄G₂₈₅T₂₈₆D₂₈₇ N₂₈₈↑ 44 NT NT NT NT H-I ↑T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 34 NT NT NT NT Q₃₂₄ M₃₂₅ T₃₁₄↑S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 71 58 65 NT 83 Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅↑K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 100  100  NT 38 36 Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆↑S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 100  NT NT NT NT Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇↑G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 59 85 NT NT NT Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈↑G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 56 100  36 97 100  Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉↑Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 83 NT NT 77 79 Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀↑A₃₂₁G₃₂₂D₃₂₃ 91 66 81 NT NT Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁↑G₃₂₂D₃₂₃ 73 62 67 34 33 Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂↑D₃₂₃ 100  60 NT 72 58 Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 100  86 99 64 NT ↑Q₃₂₄ M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ 75 70 NT 95 NT Q₃₂₄ ↑M₃₂₅ T₃₁₄ S₃₁₅K₃₁₆S₃₁₇G₃₁₈G₃₁₉Q₃₂₀A₃₂₁G₃₂₂D₃₂₃ BG BG NT NT NT Q₃₂₄ M₃₂₅↑ (% VLP: VLP formation efficiency; BG: Background expression; NT: Not Tested).

Example 2 Insertion of Peptides of Interest in P Loop Regions of VP2 of Different Lengths

To test if the length of VP2 proteins could affect VLP formation efficiency, IBDV VP2 of different lengths at their C-terminal, namely VP2 with 452, 441 and 456 amino acids (VP2 452, VP2 441 and VP2 456), were compared for their capacity to incorporate insertions of peptides of interest, namely cMyc (SEQ. ID. NO: 3) and Flag (SEQ. ID. NO: 2), in locations within P loop regions DE and HI. For the construction of pESC-URA/VP2 456 and pESC-URA/VP2 441 the VP2 cDNA was amplified using oligonucleotides VP2 452EcoRI-fw (SEQ. ID. NO: 6) and VP2 456-rev (SEQ. ID. NO: 8) or VP2 441-rev (SEQ. ID. NO: 9) and template pESC-URA/pVP2 512 which contains full-length VP2 512 insert. Purified VP2 456 and VP2 441 gene fragments were cloned into EcoRI and NotI digested plasmids pESC-URA/VP2 452. For this purpose VP2 loop insertion plasmids [e.g.: pESC-URA/VP2 (H₂₅₃↑Flag↑G₂₅₄), were digested with RsrII and MscI restriction enzymes, which cut within the VP2 gene at amino acid position G₂₄-W₄₁₄, and the purified VP2 gene fragments were cloned into RsrII and MscI digested plasmids pESC-URA/VP2 441 and pESC-URA/VP2 456. Correct clones were identified by restriction analysis and sequencing and transformed into S. cerevisiae Y449 strain to evaluate VP2 expression and VLP formation efficiency by quantitative VLP-ELISA using total yeast cell extracts.

As demonstrated in Table 2, VLP production efficiency varied according to the insertion site within VP2 as well as with VP2 length. Furthermore, it was observed that shorter VP2 in some cases resulted in improved VLP formation efficiency.

TABLE 2 VLP formation of VP2 constructs with insertion of peptides of interest in loop regions of VP2 441 and VP2 456. P-LOOP INSERTION POSITION REGION (INSERTED PEPTIDE SEQUENCE) VP2 PROTEIN LENGTH % VLP D-E H₂₅₃↑Flag↑G₂₅₄ VP2 452 100  (TS DYKDDDDKGSGGSSDYKDDDDKS TS) VP2 441 100  VP2 456 56 H-I A₃₂₁↑Flag↑ G₃₂₂ VP2 452 62 (TS DYKDDDDKGSGGSSDYKDDDDKS TS) VP2 441 67 VP2 456 56 D-E S₂₅₁↑Flag↑V₂₅₂ VP2 452 BG (TS DYKDDDDKGSGGSSDYKDDDDKS TS) VP2 441 34 VP2 456 BG D-E H₂₅₃↑cMyc↑G₂₅₄ VP2 452 84 (TS EQKLISEEDLS TS) VP2 441 83 VP2 456 54 (Peptides of interest appear underlined and flanked by TS linker sequences; BG: Background expression; % VLP: VLP formation efficiency).

Example 3 Substitution of P Loop Regions for Peptides of Interest

To facilitate substitution of P loop regions by peptides of interest, cloning vectors were generated in which the codons within the P loop regions were replaced by a sequence encoding for a short linker containing a NotI restriction enzyme site using a pESC-URA/VP2 452 plasmid with a mutation in the NotI site downstream of the VP2 452 gene (pESC-URA/VP2 452 [ΔNotI]). In one series of mutants the entire P loop region was deleted and in another series all except the first and last codon of each P loop were deleted, as shown in Table 3. Example peptides of interest, namely Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3) were cloned following standard procedures as NotI dsDNA fragments for the DNA of interest into NotI linearised cloning vectors [e.g.: pESC-URA/VP2/ΔY₂₂₀-G₂₂₃] generating in frame insertions. Purified constructs [e.g.: pESC-URA/VP2/ΔY₂₂₀-G₂₂₃/Q₂₁₉↑Flag↑G₂₂₄ were used to transform S. cerevisiae Y449 and the VLP production efficiency, shown in Table 3, was quantitatively determined by VLP-ELISA on total yeast cell extracts. Table 3 shows VLP formation efficiency when VP2 loop sequences are substituted with peptides of interest. As demonstrated in Table 3, VLP formation efficiency was generally low, but still equal or above 20%, and varied according to the site of substitution within VP2. Furthermore, it was observed that retention of the first and last residue of the P loop regions improved VLP formation efficiency.

TABLE 3 Substitutions of VP2 loop sequences with Peptide of Interest. P-LOOP SUBSTITUTION REGION POSITION INSERTED PEPTIDE (SEQUENCE) % VLP B-C S₂₁₈ ↑ . . . ↑V₂₂₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) BG cMyc (GGSGR EQKLISEEDL GGSGR) BG S₂₁₈ Q₂₁₉↑ . . . ↑G₂₂₄V₂₂₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) 24 cMyc (GGSGR EQKLISEEDL GGSGR) 33 D-E F₂₄₈↑ . . . ↑L₂₅₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) 25 cMyc (GGSGR EQKLISEEDL GGSGR) BG F₂₄₈R₂₄₉↑ . . . ↑G₂₅₄L₂₅₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) 35 cMyc (GGSGR EQKLISEEDL GGSGR) 27 F-G L₂₈₄↑ . . . ↑N₂₈₈ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) BG cMyc (GGSGR EQKLISEEDL GGSGR) BG L₂₈₄T₂₈₃↑ . . . ↑D₂₈₇N₂₈₈ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) 22 cMyc (GGSGR EQKLISEEDL GGSGR) 21 H-I T₃₁₄↑ . . . ↑M₃₂₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) BG cMyc (GGSGR EQKLISEEDL GGSGR) 25 T₃₁₄S₃₁₅↑ . . . ↑Q₃₂₄M₃₂₅ Flag (GGSGR DYKDDDDKGSGGSSDYKDDDDK GGSGR) 28 cMyc (GGSGR EQKLISEEDL GGSGR) 34 (Peptides of interest appear underlined and flanked by linker sequences; BG: Background expression; % VLP: VLP Formation Efficiency).

Example 4 Incorporation of Lysine (K) Residues in Loop Regions and Chemical Conjugation of cMvc

To generate additional conjugation sites within VP2, lysine (K) residues were cloned as substitutions of P loop region residues. Briefly, different lysine mutants were generated by site-directed mutagenesis of plasmid pESC-URA/VP2 [e.g.: pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K); mutations are expressed as the mutated amino acid followed by the induced K residue]. Purified constructs were transformed into S. cerevisiae Y449 and VP2 expression and VLP formation efficiency was determined by quantitative VLP-ELISA on total yeast cell extracts. Table 4 shows VLP production efficiency of mutants expressed as % VLP production efficiency compared the native VP2-VLP.

As shown in Table 4, mutants with 2, 3 and 4 additional lysine residues were expressed to high levels and T=1 VLP production efficiency was comparable to that of native VP2 452. Only mutant VP2(G₂₈₅K-Q₃₂₀K) made to contain a lysine residue in replacement G₂₈₅ and another in replacement of Q₃₂₀ did not form VLP. EM analysis of the K substitution constructs confirmed the presence of T=1 VLP. For the chemical conjugation VLP were purified by means of sucrose gradient centrifugation and the peptide conjugation efficiency of the native VP2 452, 3K and 4K mutant VLP were compared. VLP-peptide conjugates were prepared with 0.5-2 mg of purified VP2 VLP samples. In a first step, VLP were incubated with 3-maleimidobenzoic acid N-succinimidyl ester at a ratio of 1:50 for 30 minutes at 20° C. and subsequently dialyzed to eliminate MBS. The resulting VLP-MBS and the example peptide of interest, namely a modified cMyc (SEQ. ID. NO: 3) made to contain a terminal cysteine (C) residue, were mixed 1:50 (VLP-MBS: cMyc) and incubated over night at 4° C., pH 7.0. The final conjugation (VLP-cMyc) product was dialysed, freeze dried and the amount of conjugated peptide was quantified by ELISA using cMyc specific antibodies. Results showed clearly that the incorporation of additional K residues by means of substitutions increased conjugation efficiency to peptides of interest.

TABLE 4 Chimeric VP2 VLP incorporating Lysine substitutions in P loop regions. NUMBER OF VLP INCORPORATED B-C D-E F-G H-I MORPHOLOGY K RESIDUES Q₂₂₁K H₂₅₃K G₂₈₅K Q₃₂₀K. % VLP (EM) 0 100  T = 1 1 X NT NT 1 X NT NT 1 X NT NT 1 X X NT NT 2 X X BG T = 1 2 X X 66 T = 1 3 X X X 55 T = 1 4 X X X X 45 T = 1 (% VLP: VLP formation efficiency; BG: Background expression; NT: Not Tested).

Example 5 Identification of VP2 Insertion Sites Outside P Loop Regions by Random Transposon Mutagenesis with Tn5

Insertion of the Tn5 transposon results in the insertion of 57 nucleotides coding for 19 amino acids. Random insertion mutagenesis of IBDV VP2 with Tn5, using Plasmid pESC-URA/VP2 452 (ΔSpeI/NotI) and EZ-Tn5 In-Frame Linker Insertion Kit (Epicentre™), generated a library of DNA clones with insertions along the complete VP2 452 amino acid sequence. Transformation of competent cells was carried out using Transformax EC100 Electrocompetent cells, obtaining >200,000 clones with resistance to ampicillin (provided by the plasmid) and kanamycin (provided by the transposon). Plasmid DNA of all clones were isolated and purified to form a first Tn5 library. DNA from this first Tn5 library was digested with EcoRI and BglII to purify the band corresponding to the VP2 coding sequence with one Ez-Tn5 insertion. This band was cloned in the pESC-URA vector digested with EcoRI and BglII to generate a second library, with random insertions only in the VP2 gene. Ligation was transformed as above and 65,000 clones were obtained. Plasmid DNA of all clones were isolated and purified to form a second Tn5 library. DNA from this second Tn5 library was digested with NotI and re-ligated, to eliminate the kanamycin-resistance gene from the insertion. Religation product was transformed as above to obtain 350,000 new clones that constituted the final 19 amino acid random Tn5 insertion library. S. cerevisiae Y499 yeast cells were transformed with 10 pg of this final random Tn5 insertion library and plated into YNB/CSM-URA+2% glucose plates. 110,000 yeast clones were obtained and were transferred to galactose-containing plates. Colonies grown in the presence of galactose were transferred to a PVDF membrane to analyze VP2 VLP expression by colony immunoblot using VP2-VLP specific antibodies. A subset of positive clones were growth individually in liquid YNB/CSM-URA+2% galactose medium and VLP expression was analyzed by VLP-ELISA. Insertions within VP2 were identified by sequencing the PCR product obtained from each clone using VP2 and Tn5 specific primers. Tn5 mutagenesis led to the insertion of 19 amino acid peptides which consist of one of three possible 15 residue core peptides and 4 variable residues depending of the insertion site. Evaluation of the capacity of Tn5 constructs to produce VLP resulted in the identification of a number of insertion sites, listed in Table 5, compatible with VLP formation, and with potential for the generation of chimeric VP2 VLP with insertion or substitutions with peptides of interest.

The identification of the Tn5 insertion locations was carried out from limited analysis of 150 clones from the final random Tn5 insertion library. Other insertion locations may also be contained in said insertion library and also represent potential insertion or substitution sites for peptides of interest.

TABLE 5 VP2 insertion sites identified by random Tn5 transposon mutagenesis. INSERTION POSITION INSERTED PEPTIDE E₃₄↑K₃₅ T VSCTHLAAARCVQET ALE E₃₄↑K₃₅ L SLVHILRPQDVYKRQ TLE E₄₁↑T₄₂ T VSCTHLAAARCVQET GSE T₄₄↑Y₄₅ S VSCTHLAAARCVQET AST G₅₅↑L₅₆ L SLVHILRPQDVYKRQ GSG Q₇₅↑G₇₆ A VSCTHLAAARCVQET ALQ G₇₆↑N₇₇ L SLVHILRPQDVYKRQ LQG S₁₀₃↑R₁₀₄ L SLVHILRPQDVYKRQ LVS S₁₀₃↑R₁₀₄ R CLLYTSCGRKMCTRD RVS T₁₀₇↑V₁₀₈ V CLLYTSCGRKMCTRD STV S₁₁₁↑T₁₁₂ T VSCTHLAAARCVQET GSS T₁₁₂↑L₁₁₃ P VSCTHLAAARCVQET AST G₁₁₅↑G₁₁₆ L SLVHILRPQDVYKRQ LPG G₁₁₆↑V₁₁₇ A VSCTHLAAARCVQET AGG A₁₁₉↑L₁₂₀ P VSCTHLAAARCVQET VYA V₁₇₈↑R₁₇₉ L SLVHILRPQDVYKRQ GYV P₁₈₃↑I₁₈₄ L SLVHILRPQDVYKRQ GDP A₁₈₆↑I₁₈₇ I CLLYTSCGRKMCTRD SPA G₁₈₈↑L₁₈₉ P VSCTHLAAARCVQET AIG D₁₉₀↑P₁₉₁ P VSCTHLAAARCVQET GLD C₁₉₇↑D₁₉₈ A VSCTHLAAARCVQET ATC D₁₉₈↑S₁₉₉ T VSCTHLAAARCVQET ACD D₂₀₁↑R₂₀₂ T VSCTHLAAARCVQET GSD Y₂₀₆↑T₂₀₇ T CLLYTSCGRKMCTRD RVY S₂₄₀↑L₂₄₁ P VSCTHLAAARCVQET VTS N₄₃₃↑S₄₃₄ S VSCTHLAAARCVQET DLN N₄₃₃↑S₄₃₄ S CLLYTSCGRKMCTRD SLN A₄₅₀↑I₄₅₁ I CLLYTSCGRKMCTRD SRA R₄₅₂↑ L SLVHILRPQDVYKRQ AIR (15 residue long TN5 derived core peptides are shown underlined).

Example 6 Identification of VP2 Insertion Sites Outside P Loops by Random Transposon Mutagenesis with Mu

Insertion of Mu transposon results in the insertion of 15 nucleotides coding for a 5 amino acid peptide. The sequence of the insert varied between CGR, PRH or AAA, with two random flanking residues, depending on the insertion site and reading frame. Random insertion mutagenesis of IBDV VP2 generated a DNA library of clones with insertions along the complete VP2 452 sequence. In particular, a yeast expression plasmid pESC-URA/VP2 was used to generate a random-insertion library using the Entranceposon M1-Cam® (M1-Cam) Mutation Generation System F-701 (MGS™). Reaction was made according to manufacturer instructions and 0.5-1 μl were used to transform Transformax EC100 Electrocompetent cells, obtaining >100,000 clones with resistance to ampicillin (provided by the plasmid) and chloramphenicol (provided by the transposon). Plasmid DNA for all clones was simultaneously purified to form a first Mu library. This first Mu library was digested with EcoRI and BglII and the band corresponding to the VP2 coding sequence with one Mu insertion was isolated. This band was cloned in the pESC-URA vector digested with EcoRI and BglII to generate a second library, with random insertions only in the VP2 gene. Ligation was transformed into electrocompetent cells and >80,000 clones were obtained. Plasmid DNA for all clones was simultaneously purified to form a second Mu library. DNA from the second Mu library was digested with NotI and re-ligated, to eliminate the chloramphenicol-resistance gene from the insertion. This DNA was transformed as described above obtaining >100,000 new clones that constituted the final 5 amino acid random Mu insertion library. S. cerevisiae Y449 yeast cells were transformed with 10 μg of random Mu insertion library and plated into YNB/CSM-URA+2% glucose plates. ˜110,000 yeast clones were obtained and transferred to galactose-containing plates. Colonies grown in the presence of galactose were transferred to a PVDF membrane to analyze for VP2 expression by colony immunoblot. A subset of positive clones were growth individually in liquid YNB/CSM-URA+2% galactose medium and VLP expression was analyzed by VLP-ELISA.

Insertions were identified by sequencing the PCR product obtained from each clone using VP2 and Tn5 specific primers. Mu mutagenesis led to the insertion of 5 amino acid peptides which consist of one of three possible 3 residue core peptides and 2 variable residues depending of the insertion site. Evaluation of the capacity of Mu constructs to produce VLP resulted in the identification of a number of insertion sites, listed in Table 6, compatible with VLP formation, and with potential for the generation of chimeric VP2 VLP incorporating peptides of interest by insertion or substitution.

The identification of the Mu insertion locations was carried out from limited analysis of 200 clones from the final random Mu insertion library. Other insertion locations may also be contained in said insertion library and also represent potential insertion or substitution sites for peptides of interest.

TABLE 6 VP2 insertion sites identified by random Mu transposon mutagenesis. INSERTION POSITION INSERTED PEPTIDE M₁↑T₂ M RPH M G₅₀↑D₅₁ CGR MG Y₇₂↑T₇₃ CGR NY G₇₆↑N₇₇ AAA QG S₁₀₃↑R₁₀₄ CGR MS V₁₀₈↑R₁₀₉ AAA TV L₁₁₃↑P₁₁₄ RPH TL G₁₁₆↑V₁₁₇ CGR SG G₁₂₂↑T₁₂₃ CGR NG I₁₈₄↑P₁₈₅ CGR TI A₁₈₆↑I₁₈₇ AAA PA A₁₈₆↑I₁₈₇ CGR TA G₁₈₈↑L₁₈₉ CGR IG D₁₉₈↑S₁₉₉ AAA CD D₂₀₁↑R₂₀₂ M RPH D L₃₇₉↑A₃₈₀ RPH EL G₃₉₃↑A₃₉₄ AAA PG N₃₉₆↑Y₃₉₇ AAA MN M₄₂₇↑E₄₂₈ D AAA M L₄₃₆↑K₄₃₇ AAA PL G₄₄₀↑A₄₄₁ CGR NG F₄₄₄↑K₄₄₅ CGR SF R₄₄₉↑A₄₅₀ V RPH R (Mu derived core peptides are shown underlined).

Example 7 Cloning of Flag or cMyc Outside VP2 P Loop Regions

Removal of Tn5 or Mu transposons randomly inserted in VP2 genome resulted in inserts of 57 or 15 nucleotides encoding for a 19 or 5 amino acid insertions respectively. Resulting inserts contained a NotI restriction site that permitted cloning of DNA of interest. Resulting vectors [e.g.: pESC-URA/VP2(G₇₆↑Tn5↑TN₇₇); Insertion location of Tn5 is indicated by the arrows] were digested with NotI and ligated to a DNA of interest, such as Flag (SEQ. ID. NO: 2) or cMyc (SEQ. ID. NO: 3), with NotI sticky ends. The resulting constructs [e.g.: pESC-URA/VP2(G₇₆↑Tn5-Flag↑N₇₇)] were transformed into S. cerevisiae Y449 and the ability to form VLP, shown in Table 7, was quantitatively determined by VLP-ELISA on total yeast cell extracts.

In addition, insertion vectors were also constructed by site directed mutagenesis to generate SpeI restriction sites at previously identified transposon insertion sites. Insertion of the SpeI restriction site generated Serine-Threonine (TS) insertion at the desired pre-identified points within VP2. For insertion of DNA of interest the resulting insertion vectors [e.g.: pESC-URA/VP2(G₇₆↑TS↑N₇₇)] were digested with SpeI and ligated to a DNA of interest, such as Flag or cMyc, SpeI sticky ends. Resulting constructs [e.g.: pESC-URA/VP2(G₇₆↑TS-Flag↑N₇₇)] were transformed into S. cerevisiae Y449 and the ability to form VLP, shown in Table 7, was quantitatively determined by VLP-ELISA on total yeast cell extracts.

The insertion of peptides of interest into the Tn5 or Mu inserts generally resulted in decreased, but still equal or above 20%, VLP formation efficiency compared to the Tn5 or Mu containing constructs. However the data demonstrated the viability of the approach in identifying potential VP2 insertion or substitution locations for peptides of interest by means of transposon mutagenesis.

TABLE 7 VP2 insertions at sites outside the P loop regions. POSITION Insert INSERTED PEPTIDE % VLP Q₇₆↑N₇₇ Tn5 LSLVHILRPQDVYKRQLQG 66 Tn5-cMyc LSLVHILRP EQKLISEEDLS RPQDVYKRQLQG 45 TS-cMyc TS EQKLISEEDL TS 88 Tn5-Flag LSLVHILRP DYKDDDDKGSGGSSDYKDDDDK RPQDVYKRQLQG 55 TS-Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 77 T₁₁₂↑L₁₁₃ Tn5 PVSCTHLAAARCVQETAST 56 Tn5-cMyc PVSCTHLAA EQKLISEEDL SPAARCVQETAST 22 TS-cMyc TS EQKLISEEDL TS 71 Tn5-Flag PVSCTHLAATS DYKDDDDKGSGGSSDYKDDDDK TSSPAARCVQETAST BG TS-Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 80 A₁₁₉↑L₁₂₀ Tn5 PVSCTHLAAARCVQETVYA 116  Tn5 PVSCTHLAA EQKLISEEDL SPAARCVQETVYA 33 Tn5-cMyc TS EQKLISEEDL TS 100  TS-cMyc PVSCTHLAATS DYKDDDDKGSGGSSDYKDDDDK TSSPAARCVQETVYA 25 Tn5-Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 97 G₁₈₈↑L₁₈₉ Tn5 PVSCTHLAAARCVQETAIG 37 Tn5-cMyc PVSCTHLAA EQKLISEEDL SPAARCVQETAIG BG TS-cMyc TS EQKLISEEDL TS 43 Tn5-Flag PVSCTHLAATS DYKDDDDKGSGGSSDYKDDDDK TSSPAARCVQETAIG 25 TS-Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 43 I₁₈₄↑P₁₈₅ Mu I CGR TIP 65 Mu-cMyc ICGP TEQKLISEEDLS GRTIP 56 cMyc TS TEQKLISEEDLS TS 63 Mu-Flag ICGP DYKDDDDKGSGGSSDYKDDDDKS GRTIP 64 Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 70 I₃₇₉↑A₃₈₀ Mu L RPH ELA 65 Mu-cMyc LRP TEQKLISEEDLS RPHELA 56 cMyc TS TEQKLISEEDLS TS 67 Mu-Flag LRP DYKDDDDKGSGGSSDYKDDDDKS RPHELA 56 Flag TS DYKDDDDKGSGGSSDYKDDDDK TS 63 (Inserted peptides of interest are shown underlined with the reminder amino acids resulting from the inserted Tn5 or Mu transposon, or the engineered TS site; BG: Background expression; % VLP: VLP formation efficiency).

Example 8 Chimeric VP2 VLP Incorporating Multiple VP2 Insertions

In order to evaluate the capacity of VP2 VLP to incorporate peptide insertions at two different sites, a series of constructs were generated as shown in FIG. 4. Plasmids pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄) and pESC-URA/VP2(H₂₅₃↑cMyc↑G₂₅₄) were digested with restriction enzymes RsrII and NatI. The DNA fragments encoding for N-terminal part of VP2 including the insertions at H₂₅₃↑G₂₅₄ were purified and cloned into the RsrII, NarI digested plasmids pESC-URA/VP2(A₃₂₁↑Flag↑G₃₂₂) and pESC-URA/VP2(A₃₂₁↑cMyc↑G₃₂₂). The resulting plasmids, namely, pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄/A₃₂₁↑Flag↑G₃₂₂), pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄/A₃₂₁↑cMyc↑G₃₂₂), pESC-URA/VP2 (H₂₅₃↑cMyc↑G₂₅₄/A₃₂₁↑Flag↑G₃₂₂), and pESC-URA/VP2(H₂₅₃↑cMyc↑G₂₅₄/A₃₂₁↑cMYc↑G₃₂₂), were used to transform S. cerevisiae Y449 strain to evaluate VP2 expression and chimeric VP2 VLP formation capacity by quantitative VLP-ELISA. As shown in Table 8, VP2 452 proteins with two P loop region insertions are capable of forming VLP. In general it was observed that the second insertion reduced VLP formation efficiency slightly but VLP were formed in all cases with efficiencies equal or above 20%, and the presence of VLP was confirmed by EM analysis.

TABLE 8 VLP production capacity of VP2 constructs with multiple insertions. INSERTION POSITION INSERTION POSITION (INSERTED PEPTIDE SEQUENCE) (INSERTED PEPTIDE SEQUENCE) % VLP H₂₅₃↑Flag↑G₂₅₄ none 100 (TSDYKDDDDKGSGGSSDYKDDDDKTS) A₃₂₁↑Flag↑G₃₂₂ 66 (TSDYKDDDDKGSGGSSDYKDDDDKTS) A₃₂₁↑cMyc↑G₃₂₂ 45 (TSEQKLISEEDLTS) H₂₅₃↑cMyc↑G₂₅₄ none 84 (TSEQKLISEEDLTS) A₃₂₁↑Flag↑G₃₂₂ 33 (TSDYKDDDDKGSGGSSDYKDDDDKTS) A₃₂₁↑cMyc↑G₃₂₂ 82 (TSEQKLISEEDLTS) none A₃₂₁↑Flag↑G₃₂₂ 62 (TSDYKDDDDKGSGGSSDYKDDDDKTS) A₃₂₁↑cMyc↑G₃₂₂ 67 (TSEQKLISEEDLTS) (Core peptides are shown in bold and underlined; % VLP: VLP Formation Efficiency).

Example 9 Chimeric VP2 VLP Resulting from Chimeric VP2 Fusion Proteins Incorporating the Insertion or Substitution with a Peptide of Interest in Addition to a Terminally Fused Peptide of Interest

To test the possibility of combining VP2 insertions or substitutions with terminal fusions, a series of VP2 insertion or substitution constructs with N- or C-terminal peptide fusions were prepared as shown in FIG. 5. For the construction of a C-terminal fusion site DNA constructs incorporating the insertion of a DNA of interest, Flag or cMyc, within VP2 at the H₂₅₃↑G₂₅₄ or A₃₂₁↑G₃₂₂ site and DNA constructs encoding for a 4K substitution mutant, containing four lysine residues in P loop regions, were genetically engineered to contain unique NotI and HindIII restriction sites at the C-terminal end of the VP2 gene [e.g.: pESC-URA/VP2(H₂₅₃↑cMyc↑G₂₅₄)-Not Hind; and pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-Not Hind]. Double digest of the VP2 insertion or substitution vectors with NotI-HindIII and re-ligation to incorporate a NotI-HindIII linear fragment encoding for cMyc or Flag, resulted in the C-terminal fusion of the second, or same, DNA of interest or in the combination of the C-terminal fusion and 4K substitutions [e.g.: pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄)-Flag; or pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-cMyc].

For the construction of N-terminal fusions DNA constructs incorporating the insertion of a DNA of interest, Flag (SEQ. ID. NO: 2) or cMyc (SEQ. ID. NO: 3), within VP2 at the H₂₅₃↑G₂₅₄ or A₃₂₁↑G₃₂₂, H₂₅₃↑G₂₅₄ site and DNA constructs encoding for the 4K substitution mutant were digested with EcoRI, positioned immediately upstream of the start-codon, and ligated to a Flag encoding DNA fragment with EcoRI sticky ends. Transformation of the ligation product into supercompetent E. coli led to the isolation plasmids encoding mutants with N-terminal Flag fusions in combination with VP2 insertions or substitutions. [e.g.: pESC-URA/Flag-VP2(H₂₅₃↑Flag↑G₂₅₄); or pESC-URA/Flag-VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)]. Transformation of all constructs into S. cerevisiae Y449 and isolation of transformed yeast colonies was followed by evaluation of VLP production efficiency, as shown in Table 11. In general fusion of a peptide of interest to the C-terminus reduced VLP formation efficiency but VLP were formed in all cases with efficiencies equal or above 20%, and the presence of VLP was confirmed by electron microscopy (EM).

TABLE 9 VLP production capacity of VP2 constructs incorporating the insertion of a peptide of interest and a C- or N-terminal fusion of a second peptide of interest. INSERTION POSITION TERMINAL FUSION POSITION (INSERTED PEPTIDE SEQUENCE) (INSERTED PEPTIDE SEQUENCE) % VLP H₂₅₃↑Flag↑G₂₅₄ None 100  (TSDYKDDDDKGSGGSSDYKDDDDKSTS) R₄₅₂↑Flag (SSGDYKDDDDK) 23 R₄₅₂↑cMyc (SSGDYKDDDDK) 30 Flag↑M₁ (MDYKDDDDKGNSEF) 45 A₃₂₁↑Flag↑G₃₂₂ None 62 (TSDYKDDDDKGSGGSSDYKDDDDKSTS) R₄₅₂↑Flag (SSGDYKDDDDK) 67 R₄₅₂↑cMyc (SSGDYKDDDDK) NT Flag↑M₁ (MDYKDDDDKGNSEF) 34 H₂₅₃↑cMyc↑G₂₅₄ None 84 (TSEQKLISEEDLSTS) R₄₅₂↑Flag (SSGDYKDDDDK) 55 R₄₅₂↑cMyc (SSGDYKDDDDK) 30 Flag↑M₁ (MDYKDDDDKGNSEF) 78 A₃₂₁↑cMyc↑G₃₂₂ None 67 (TSEQKLISEEDLTSTS) R₄₅₂↑Flag (SSGDYKDDDDK) 85 R₄₅₂↑cMyc (SSGDYKDDDDK) NT Flag↑M₁ (MDYKDDDDKGNSEF) 56 Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K None 45 (4K) R₄₅₂↑Flag (SSGDYKDDDDK) 34 R₄₅₂↑cMyc (SSGDYKDDDDK) 40 Flag↑M₁ (MDYKDDDDKGNSEF) 55 (% VLP: VLP Formation Efficiency; BG: Background expression; NT: Not Tested)

Example 10 Production in Baculovirus of T=13 VLP Incorporating VP2 Insertions and VP3 Terminal Fusions by Expression Chimeric pVP2-VP4-VP3 Poly-Protein Genes

For the generation of IBDV T=13 VLP incorporating insertions of peptides of interest within VP2, plasmids were constructed for the expression of the IBDV pVP2-VP4-VP3 polyprotein. More specifically, baculovirus expression plasmids, namely pFastBacDual (pFBD) from Invitrogen™, and synthetic pVP2 (SEQ. ID. NO: 21), VP4 (SEQ. ID. NO:23) and VP3 (SEQ. ID. NO: 25) genes of IBDV Soroa strain cloned into pUC57 (NCIB No. AAD30136) as shown in FIGS. 6, 7 and 8, were used for the construction pFBD/pVP2-VP4-VP3 polyproteins plasmids [i.e.: pFBD/pVP2-VP4-VP3-pp]. Construction of plasmids is outlined in FIG. 9 and as follows:

In a first step, the gene encoding for the full length 512 amino acid pVP2 (SEQ. ID. NO: 21) was the cloned into pFBD plasmid downstream of the PH promoter. Plasmid pUC57-pVP2 was digested with restriction enzymes BglII and HindIII and the DNA fragment was cloned into BamHI and HindIII digested pFBD plasmid generating pFBD/pVP2-pp.

In a second step the VP3 fusion proteins genes were cloned into pFBD/VP2 downstream of the pVP2 gene. DNA fragments encoding for VP3-Flag and VP3-cMyc were obtained by PCR using the synthetic VP3 gene (pUC57-VP3) shown in FIG. 8 as template and oligonucleotide primers BV VP3 5′ HindIII (SEQ. ID. NO: 10) and BV VP3-Flag 3′HindIII (SEQ. ID. NO: 11), BV VP3-cMyc 3′HindIII (SEQ. ID. NO: 12) respectively as shown in FIG. 9. Purified DNA fragments were digested with restriction enzymes HindIII and cloned into HindIII digested pFBD/pVP2-pp generating pFBD/pVP2-VP3-Flag-pp and pFBD/pVP2-VP3-cMyc-pp.

In a third step, DNA fragments encoding for the VP2 insertions were cloned into pFBD/pVP2-VP3(X)-pp plasmids. For this purpose loop insertion plasmids [e.g.: pFBD/VP2(H₂₅₃↑Flag↑G₂₅₄); pFBD/VP2(H₂₅₃↑cMYc↑G₂₅₄); or pFBD/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)] were digested with the restriction enzyme RsrII and NcoI, which cut within the VP2 gene at amino acid positions G₂₄-H₃₃₈ and the purified VP2 gene fragment was cloned into RsrII and NcoI digested pFBD/pVP2-VP3-Flag-pp and pFBD/pVP2-VP3-cMyc-pp plasmids, generating plasmids pFBD/pVP2(H₂₅₃↑Flag or Myc↑G₂₅₄)-VP3-Flag-pp, pFBD/pVP2(H₂₅₃↑Flag or Myc↑G₂₅₄)-VP3-cMyc-pp, pFBD/pVP2(A₃₂₁↑Flag or Myc↑G₃₂₂)-VP3-Flag-pp, pFBD/pVP2(A₃₂₁↑Flag or Myc↑G₃₂₂)-VP3-cMyc-pp, pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-VP3-Flag-pp and pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀-K)-VP3-cMyc-pp, as shown in FIG. 9.

In a final step of the construction the VP4 gene was inserted between the pVP2 and VP3 genes to create the pVP2-VP4-VP3 open reading frame. To carry this out, plasmid pUC57-VP4 was digested with restriction enzymes PvuII and BsgI and the pVP4 DNA fragment was cloned into PvuII and Bsgl digested plasmids pFBD/pVP2(X)-VP3-X (e.g. pFBD/pVP2(H₂₅₃↑Flag or Myc↑G₂₅₄)-VP3-Flag-pp and pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀-K)-VP3-cMyc-pp)

The resulting plasmids [e.g.: pFBD/pVP2(H₂₅₃↑Flag or Myc↑G₂₅₄)-VP4-VP3-Flag-pp and pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀-K)-VP4-VP3-cMyc-pp] were introduced in recombinant Baculoviruses (rBV) which in the course of their replicating cycle expressed the pVP2(X)-VP4-VP3-X polyproteins. VLP production in rBV infected insect cells and T=13 VLP purification was carried out following standard procedures.

Briefly, cultures of H5 insect cells (Invitrogen™) were infected with rBV at a multiplicity of infection of 5 pfu/cell. At 30 h post infection, cells were harvested, lysed, and VLP purified by sucrose gradient centrifugation. T13 VLP formation capacity for all constructs was evaluated by VLP-ELISA and EM of purified VLP samples. As shown in Table 10, all tested constructs resulted in T=13 VLP, indicating that chimeric VP2 fusion proteins containing insertions are indeed compatible with T=13 VLP formation and furthermore that T=13 VLP can be made to incorporate the combination of peptides of interests inserted in VP2 and fused to VP3.

TABLE 10 VLP production capacity of T = 13 VLP incorporating VP2 insertions and VP3 terminal fusions and produced by expression pVP2-VP4-VP3 poly-protein gene in baculovirus. INSERTION POSITION POSITION/PEPTIDE (INSERTED PEPTIDE SEQUENCE) VP3 fusion SEQUENCES % VLP* H₂₅₃↑Flag↑G₂₅₄ VP3-Flag E₂₅₉/SSGDYKDDDDK 80 (TSDYKDDDDKGSGGSSDYKDDDDKTS) VP3-cMyc E₂₅₉/EQKLISEEDL 92 H₂₅₃↑CMyc↑G₂₅₄ VP3-Flag E₂₅₉/SSGDYKDDDDK 77 (TSEQKLISEEDLTS) VP3-cMyc E₂₅₉/EQKLISEEDL 55 A₃₂₁↑Flag↑G₃₂₂ VP3-Flag E₂₅₉/SSGDYKDDDDK 44 (TSDYKDDDDKGSGGSSDYKDDDDKTS) VP3-cMyc E₂₅₉/EQKLISEEDL 77 A₃₂₁↑CMYc↑G₃₂₂ VP3-Flag E₂₅₉/SSGDYKDDDDK 82 (TSEQKLISEEDLTS) VP3-cMyc E₂₅₉/EQKLISEEDL 76 Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K (4K) VP3-Flag E₂₅₉/SSGDYKDDDDK 66 VP3-cMyc E₂₅₉/EQKLISEEDL 45 (*% VLP formation efficiency compared to pVP2-HisVP3 generated by co-expression pVP2 and VP3).

Example 11 Production in Yeast S. cerevisiae of T=13 VLP Incorporating a VP2 Insertion and VP3 Terminally Fused to N- and C-Terminus

For the generation of T=13 VLP incorporating insertions of peptides of interest within VP2, plasmids were constructed for the co-expression of VP2 and VP3 from separate genes. Single expression plasmids were used, namely pESC-URA from Stratagene™, that permitted expression in yeast. Synthetic genes for the pVP2-512 (SEQ. ID. NO: 21) and VP3 (SEQ. ID. NO: 25) of IBDV Soroa strain were cloned into pUC57 (NCIB No. AAD30136) as shown in FIGS. 6 and 8, and were used for the plasmid constructions. Construction of plasmids is outlined in FIG. 10.

In a first step, the construction of pESC-URA/pVP2-VP3 plasmids VP3 fusion proteins genes were cloned into pESC-URA downstream of the GAL10 promoter (MSC1). DNA fragments encoding for His-VP3, His-VP3-Flag and His-VP3-cMyc were obtained by PCR using the synthetic VP3 gene (pUC57-VP3) shown in FIG. 8 as template and oligonucleotide primers Y His-VP3 5′ (SEQ. ID. NO: 13) and Y VP3 3′ (SEQ. ID. NO: 14), Y VP3-Flag 3′ (SEQ. ID. NO: 15), Y VP3-cMyc 3 (SEQ. ID. NO: 16), respectively as shown in FIG. 1. Purified DNA fragments were digested with restriction enzymes NotI and SacI and cloned into NotI and SacI digested pESC-URA generating the following pESC-URA/His-VP3(X) plasmids, pESC-URA/His-VP3, pESC-URA/His-VP3-Flag and pESC-URN His-VP3-cMyc.

In a second step, the gene encoding for the full length 512 amino acid pVP2 was the cloned into pESC-URA/His-VP3-X plasmids, were X is example DNA of interest, namely cMyc (SEQ. ID. NO: 3) or Flag (SEQ. ID. NO: 2), downstream of the GAL1 promoter (MCS2). Plasmid pUC57-pVP2 was digested with restriction enzymes BglII and HindIII and the DNA fragment was cloned into BamHI and HindIII digested pESC-URA/His-VP3-X plasmids, generating the following pESC-URA/pVP2-VP3-X plasmids, pESC-URA/pVP2-His-VP3, pESC-URA/pVP2-His-VP3-Flag, and pESC-URA/pVP2-His-VP3-cMyc.

In a third step, DNA fragments encoding for the VP2 insertions were cloned into pESC-URA/pVP2-His-VP3-X plasmids. For this purpose loop insertion plasmids [e.g.: pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄), or pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)] were digested with the restriction enzymes RsrII and MscI, which cut within the VP2 gene at amino acid positions G₂₄-W₄₁₄ and the purified VP2 gene fragment was cloned into RsrII and MscI digested pESC-URA/pVP2-His-VP3-X plasmids.

The resulting plasmids, namely pESC-URA/pVP2(H₂₅₃↑Flag or cMyc↑G₂₅₄)-His-VP3, pESC-URA/pVP2(A₃₂₁↑Flag or cMyc↑G₃₂₂)-His-VP3, pESC-URA/pVP2(H₂₅₃↑Flag or cMyc↑G₂₅₄)-His-VP3-Flag, pESC-URA/pVP2(H₂₅₃↑Flag or cMyc↑G₂₅₄)-His-VP3-cMyc, pESC-URA/pVP2(A₃₂₁↑Flag or cMyc↑G₃₂₂)-His-VP3-Flag, pESC-URA/pVP2(A₃₂₁↑Flag or cMyc↑G₃₂₂)-His-VP3-cMyc, pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-His-VP3, pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-His-VP3-Flag and pESC-URA/VP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-His-VP3-cMyc , were used to transform S. cerevisiae Y449 to evaluate for VP2NP3 expression and VLP formation capacity by quantitative VLP-ELISA using total yeast cell extracts. The formation of T=13 was confirmed by EM of purified VLP samples and compared to VLP formation of a pVP2-His-VP3 construct lacking VP2 insertions. As shown in Table 11 all tested constructs resulted in the formation of T=13 VLP with varying degrees of efficiency above 20% indicating that chimeric VP2 fusion proteins containing insertions are indeed compatible with T=13 VLP formation in yeast, and furthermore that T=13 VLP can be made to incorporate chimeric VP2 fusion proteins and chimeric VP3.

TABLE 11 VLP production capacity of T = 13 VLP incorporating VP2 insertions and VP3 terminal fusions generated by expression of pVP2 and VP3 in yeast. INSERTION POSITION (INSERTED PEPTIDE SEQUENCE) VP3fusion POSITION/PEPTIDE SEQUENECES % VLP* H₂₅₃↑Flag↑G₂₅₄ His-VP3 His-VP3:M₁/HHHHHHSSG 44 (TSDYKDDDDKGSGGSSDYKD His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 23 DDDKSTS) His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 12 H₂₅₃↑cMyc↑G₂₅₄ His-VP3 His-VP3:M₁/HHHHHHSSG 66 (TSEQKLISEEDLSTS) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 62 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 44 A₃₂₁↑Flag↑G₃₂₂ His-VP3 His-VP3:M₁/HHHHHHSSG 53 (TSDYKDDDDKGSGGSSDYKD His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 42 DDDKSTS) His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 66 A₃₂₁↑cMyc↑G₃₂₂ His-VP3 His-VP3:M₁/HHHHHHSSG 22 (TSEQKLISEEDLSTS) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 12 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 59 Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K His-VP3 His-VP3:M₁/HHHHHHSSG 55 (4K) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 33 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 56 (*% VLP formation efficiency compared to pVP2-HisVP3 generated by co-expression pVP2 and VP3).

Example 12 Expression of Chimeric VP2 Fusion Proteins and VP3 in Baculovirus for Production of T=13 VLP Incorporating VP2 Insertions and VP3 Terminal Fusions

For the generation of IBDV T=13 VLP incorporating insertions of peptides of interest within VP2, plasmids were constructed for the co-expression of IBDV VP2 and VP3 from separate genes. Single expression plasmids were used, namely pFastBacDual (pFBD) from Invitrogen™, which permitted expression in insect cells. Synthetic genes for the pVP2 512 (SEQ. ID. NO: 21) and VP3 (SEQ. ID. NO: 25) of IBDV Soroa strain cloned into pUC57 (NCIB No. AAD30136) as shown in FIGS. 6 and 8, were used for the plasmid constructions (see FIG. 11) as follows:

In a first step in the construction of pFBD/pVP2-VP3 plasmids, VP3 fusion proteins genes were cloned into pFBD downstream of the p10 promoter. DNA fragments encoding for His-VP3, His-VP3-Flag and His-VP3-cMyc were obtained by PCR using the synthetic VP3 gene (pUC57-VP3) shown in FIG. 6 as template and oligonucleotide primers BV His-VP3 5′ (SEQ. ID. NO: 17) and BV VP3 3′ (SEQ. ID. NO: 18), BV VP3-Flag 3′ (SEQ. ID. NO: 19), BV VP3-cMyc 3′ (SEQ. ID. NO: 20), respectively as shown in FIG. 2.

Purified DNA fragments were digested with restriction enzymes SmaI and KpnI and cloned into SmaI and KpnI digested pFBD generating pFBD/His-VP3-X plasmids, were X is DNA of interest, namely Flag (SEQ. ID. NO: 2) of cMyc (SEQ. ID. NO: 3).

In a second step, the gene encoding for the full length 512 amino acid VP2 was cloned into pFBD/His-VP3-X plasmids downstream of the PH promoter. Plasmid pUC57-pVP2 was digested with restriction enzymes BglII and HindIII and the DNA fragment was cloned into BamHI and HindIII digested pFBD/VP3-X plasmids generating the following pFBD/pVP2-His-VP3-X plasmids, pFBD/pVP2-His-VP3, pFBD/pVP2-His-VP3-Flag, and pFBD/pVP2-His-VP3-cMyc.

In a third step, DNA fragments encoding for the VP2 insertions were cloned into pFBD/pVP2-VP3-X plasmids. For this purpose loop insertion plasmids [e.g.: pESC-URA/VP2(H₂₅₃↑Flag↑G₂₅₄); pESC-URA/VP2(H₂₅₃↑cMyc↑G₂₅₄ or pESC-URA/VP2/Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K] were digested with the restriction enzyme RsrII and NcoI, which cut within the VP2 gene at amino acid positions G₂₄-H₃₃₈ and the purified VP2 gene fragment was cloned into RsrII and NcoI digested pFBD/pVP2-His-VP3-X plasmids.

The resulting plasmids, shown in FIG. 11, namely pFBD/pVP2(H₂₅₃↑Flag or cMyc↑G₂₅₄)-His-VP3, pFBD/pVP2(H₂₅₃↑Flag or cMyc↑G₂₅₄)-His-VP3-Flag, pFBD/pVP2(H₂₅₃↑Flag or Myc↑G₂₅₄)-His-VP3-cMyc, pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-His-VP3, pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-VP3-Flag and pFBD/pVP2(Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K)-His-VP3-cMyc, were introduced in recombinant Baculoviruses (rBV) which in the course of their replicating cycle expressed both proteins, VP2 fusions and VP3 fusions, simultaneously. The same procedure was carried out with Flag and cMyc VP2 insertions at location A₃₂₁↑G₃₂₂ as shown in FIG. 11. VLP production in rBV infected insect cells and T=13 VLP purification was carried out following standard procedures. Briefly, cultures of H5 insect cells (Invitrogen™) were infected with rBV at a multiplicity of infection of 5 pfu/cell. At 30 hr post infection, cells were harvested, lysed, and VLP purified by sucrose gradient centrifugation. T13 VLP formation capacity for all constructs was evaluated by ELISA and TEM of purified VLP samples and compared to VLP formation of a pVP2-His-VP3 construct lacking VP2 insertions (see FIG. 12). As shown in Table 12, all tested constructs resulted in T=13 VLP with formation efficiencies above 20%, indicating that VP2 insertions are indeed compatible with T=13 VLP formation in Baculovirus infected cells and furthermore that T=13 VLP can be made to incorporate the combination of oeptidps of interests inserted in V=P2 and fused to VP3.

TABLE 12 VLP production capacity of T = 13 VLP incorporating VP2 insertions and VP3 terminal fusions generated by co-expression of pVP2 and VP3. INSERTION POSITION (INSERTED PEPTIDE SEQUENCE) VP3 fusion POSITION/PEPTIDE SEQUENECES % VLP* H₂₅₃↑Flag↑G₂₅₄ His-VP3 His-VP3:M₁/HHHHHHSSG 66 (TSDYKDDDDKGSGGSSDYKDDDD His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 45 KSTS) His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 35 H₂₅₃↑cMyc↑G₂₅₄ His-VP3 His-VP3:M₁/HHHHHHSSG 78 (TSEQKLISEEDLSTS) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 55 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 34 A₃₂₁↑Flag↑G₃₂₂ His-VP3 His-VP3:M₁/HHHHHHSSG 78 (TSDYKDDDDKGSGGSSDYKDDDD His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 22 KSTS) His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 88 A₃₂₁↑cMyc↑G₃₂₂ His-VP3 His-VP3:M₁/HHHHHHSSG 33 (TSEQKLISEEDLSTS) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 56 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 88 Q₂₂₁K-H₂₅₃K-G₂₈₅K-H₃₂₀K His-VP3 His-VP3:M₁/HHHHHHSSG 55 (4K) His-VP3-Flag M₁/HHHHHHSSG, E₂₅₉/SSGDYKDDDDK 33 His-VP3-cMyc M₁/HHHHHHSSG, E₂₅₉/EQKLISEEDL 56 (*% VLP formation efficiency compared to pVP2-HisVP3 generated by co-expression pVP2 and VP3).

Example 13 Array Screen for Insertions Against a Library Incorporating Previously Identified VP2 Insertion Vectors

To limit the number of false positive hits during the screens, a second generation of VP2 loop insertion vectors was prepared to contain a multiple cloning site (MCS) for the efficient directional cloning of a DNA of interest. For this purpose vectors containing a SpeI site at each of the possible VP2 insertion positions were digested with SpeI and re-ligated to incorporate a MCS insert which allowed the directional cloning of DNA of interest using restriction sites NotI and SpeI (See FIG. 13). The MCS insert contained several in-frame stop codons to ensure that re-ligation of the MSC containing vectors encoded for a deleted VP2 protein unable to assemble in the form of VLP.

For the performed array test screen a selection of 7 VP2 MCS insertion vectors were plated in the form of an array, one vector per well, as triplicates in 96-well plates. The selection of vectors included constructs with insertion sites at VP2 P loop regions or outside said VP2 P loop regions. Vectors were simultaneously digested with NotI and SpeI for 2 hr in appropriate buffer conditions. Following heat inactivation at 65° C. for 30 minutes, a purified DNA fragment of the peptide of interest, namely Flag (SEQ. ID. NO: 2) and cMyc (SEQ. ID. NO: 3), with NotI and SpeI sticky ends was added and ligation performed with T4 ligase at 16° C. for 18 hr. To each of the wells 200 □l of supercompetent S. cerevisiae Y449 were added for transformation followed after 30 minutes by addition of 750 ul of selective culture medium. After cultivation at 30° C. for 16 hr, cells were lysed by 3 freeze-thaw cycles and VLP formation efficiency was tested by quantitative VLP-ELISA. VLP expression efficiency for each of the clones varied for the different vectors according to the site of insertion as shown in Table 13.

The VLP formation efficiency for each construct was calculated as an average value of the triplicate wells. The data obtained for the insertion vectors H₂₅₃↑G₂₅₄, F₂₄₉↑R₂₅₀, T₂₈₆↑D₂₈₇, D₃₂₃↑Q₃₂₄ and G₁₈₈↑L₁₈₉ agreed well with VLP formation data previously obtained for these vectors (see table 1), indicating that the array screen can accurately predict accurately the optimal insertion site for peptide of interest and that array screens are indeed a useful tool for the identification of preffered insertion site for a particular peptide of interest.

TABLE 13 VLP expression following insertion of a peptide of interest into VP2 insertion vectors arranged in an array format. (% VLP: VLP Formation Efficiency) % VLP LOCATION/ cMyc WELL POSITION DESCRIPTION Flag peptide peptide 1 H₂₅₃↑G₂₅₄ Surface P loop DE 88 85 2 F₂₄₉↑R₂₅₀ Surface P loop DE BG 25 3 T₂₈₆↑D₂₈₇ Surface P loop FG 70 45 4 D₃₂₃↑Q₃₂₄ Surface P loop HI 55 85 5 Q₇₆↑N₇₇ Outside P loop 77 54 6 S₁₁₁↑T₁₁₂ Outside P loop 55 67 7 G₁₈₈↑L₁₈₉ OutsideP loop 34 45

Example 14 Pool Screen for Flag Insertions of Against a Library Incorporating Previously Identified VP2 Insertion Vectors

For the pool screen, an equimolar selection of plasmid DNA of 7 previously selected VP2 insertion vectors containing a MCS, including a NotI/SpeI cloning site at the desired insertion site, were mixed. The selection of vectors included P loop and outside P loop region cloning sites as shown in Table 14. Pooled vectors were digested with NotI and SpeI for 2 hr in appropriate buffer conditions, incubated at 65° C. for 30 minutes and purified by gel electrophoresis. A linear fragment, with NotI and SpeI sticky ends, of the DNA of interest encoding for Flag (SEQ. ID. NO: 2) as an example peptide of interest, was mixed with the pool of digested vectors and ligated with T4 ligase at 16° C. for 18 hr. Transformation of electrocompetent E. coli with 1 ul of ligation mixture resulted in a VP2 insertion library for the peptide of interest containing insertion at the 10 different pre-selected locations. Pooled plasmid DNA was purified And 10 μg transformed into S. cerevisiae Y449 yeast and plated on YNB/CSM-URA+2% glucose. Isolated yeast clones were transferred to galactose-containing plates and resulting individual colonies were transferred to a PVDF membrane to analyze for VLP production by colony immunoblot against VP2 VLP specific antibodies. A selection of 50 positive clones were picked and grown individually in selective liquid medium (YNB/CSM-URA+2% galactose) and quantitative VLP-ELISA was carried out to confirm VLP production of positive clones. Twenty colonies with VLP formation efficiencies above 50% were sequenced to identify the site of insertion within VP2. The confirmed positive clones had insertions at sites previously identified as preferred for insertions [e.g.: H₂₅₃↑G₂₅₄, D₃₂₃↑Q₃₂₄↑Q₇₆↑N₇₇)] shown in Table 14. On the other hand, no clones with insertions at a less preferred insertion site [e.g.: F₂₄₉↑R₂₅₀] were identified in the pool screen, indicating that the array screen could predict accurately the optimal insertion site for a peptide of interest and that pool screens are indeed a useful tool for the identification of preferred insertion sites for a particular peptide of interest.

TABLE 14 Preferred VP2 VLP insertion sites for Flag resulting from a screen against a pool of 7 previously identified VP2 insertion vectors. RESULT OF POOL SCREEN/ Number of identified clones with VLP LOCATION formation efficiency >50% of VP2 452 POSITION DESCRIPTION control H₂₅₃↑G₂₅₄ P loop DE 6 F₂₄₉↑R₂₅₀ P loop DE 0 T₂₈₆↑D₂₈₇ P loop FG 2 D₃₂₃↑Q₃₂₄ P loop HI 8 Q₇₆↑N₇₇ Outside P loop 4 S₁₁₁↑T₁₁₂ Outside P loop 3 G₁₈₈↑L₁₈₉ OutsideP loop 1

Example 15 Random Transposon-Based Peptide Insertion Screen

A VP2 452 library containing random insertions of Mu transposon along the entire VP2 452 was digested with NotI at the unique site provided by the Mu transposon. The linearised plasmid pool was re-ligated to incorporate three DNA adapters with NotI sticky ends encoding for the Flag (SEQ. ID. NO: 2) or cMyc (SEQ. ID. NO: 3) peptide in each of the three possible reading frames. 0.5-1 μl of the ligation mixture was used to transform Transformax EC100 Electrocompetent E. coli to obtain a random VP2 Flag insertion library in E. coli. The library was expanded and pooled plasmid DNA purified and 10 pg used to transform S.cerevisiae Y449 yeast prior to plating onto YNB/CSM-URA+2% glucose plates. 10,000 yeast clones were obtained and were transferred to galactose-containing selection plates. Colonies grown in the presence of galactose were transferred to a PVDF membrane to analyze for VP2 expression and VLP formation by colony immunoblot. A selection of 180 positive clones were growth individually in selective liquid medium (YNB/CSM-URA+2% galactose) and quantitative VLP-ELISA was carried out to confirm VLP production of positive clones. 16 samples of the selected positive clones of the Flag and cMyc screen showed VLP formation efficiencies above 50% of the VP2 452 control and the chimeric VP2 gene of these clones was sequenced to identify the site of insertion. Table 15 shows VLP production capacity of Flag peptide VP2 insertions resulting from a random Transposon based peptide insertion screen.

As shown in Table 15, some of the confirmed positive clones contained the insertion of the peptide of interest, Flag (SEQ. ID. NO: 2), at previously identified preferred sites such as M₁↑T₂, I₁₈₄↑P₁₈₅, G₁₈₈↑L₁₈₉, L₄₃₆↑K₄₃₇ while others showed insertions at previously unidentified locations outside the P loop regions. Two clones at insertion positions I₁₈₄↑P₁₈₅, L₃₇₉↑A₃₈₀ showed the insertion of two copies indicating that at these could be preferred insertions site for large inserts. Three independent Flag insertion clone were found at two previously identified insertion site (I₁₈₄↑P₁₈₅, L₄₃₆↑K₄₃₇) suggesting that these sites constitute a preferred insertion site outside the P loop region.

TABLE 15 VLP production capacity of Flag peptide VP2 insertions resulting from a random Transposon based peptide insertion screen. INSERTION POSITION INSERTED SEQUENCE % VLP M₁↑T₂ MMRP DYKDDDDKGSGGSSDYKDDDDKS RPHMT 77 Q₉↑Q₁₀ QCGP DYKDDDDKGSGGSSDYKDDDDKS GRTQQ 79 I₁₈₄↑P₁₈₅ SCGP DYKDDDDKGSGGSSDYKDDDDKSGPDYKDDDDKGSGGSSDYKDDD 87 DKS GRTI I₁₈₄↑P₁₈₅ ICGP DYKDDDDKGSGGSSDYKDDDDKS GRTIP 63 I₁₈₄↑P₁₈₅ ICGP DYKDDDDKGSGGSSDYKDDDDKS GRTIP 54 G₁₈₈↑L₁₈₉ GCGP DYKDDDDKGSGGSSDYKDDDDKS GRIGL 85 A₂₁₁↑D₂₁₂ AAA DYKDDDDKGSGGSSDYKDDDDKS PAAAAD 59 D₂₈₇↑N₂₈₈ DAA DYKDDDDKGSGGSSDYKDDDDKS PAATDN 52 G₃₂₂↑D₃₂₃ GAA DYKDDDDKGSGGSSDYKDDDDKS GAAGD 33 L₃₇₉↑A₃₈₀ LRP DYKDDDDKGSGGSSDYKDDDDKSRPDYKDDDDKGSGGSSDYKDDDDK 39 S RPHELA L₄₃₆↑K₄₃₇ LNAA DYKDDDDKGSGGSSDYKDDDDKS RPHLK 67 L₄₃₆↑K₄₃₇ LMRP DYKDDDDKGSGGSSDYKDDDDKS PHLK 77 L₄₃₆↑K₄₃₇ LNAA DYKDDDDKGSGGSSDYKDDDDKS RPHLK 57 F₄₄₄↑K₄₄₆ FCGP DYKDDDDKGSGGSSDYKDDDDKS SRSFK 70 D₄₄₄↑I₄₄₅ DAA DYKDDDDKGSGGSSDYKDDDDKS AKDI 100 I₄₄₇↑I₄₄₈ IAA DYKDDDDEGSGGSSDYKDDDDKS PAADII 88 (Inserted peptides of interest are shown underlined with the reminder amino acids resulting from the inserted Mu transposon; % VLP: VLP formation efficiency). 

1. A fusion protein, comprising an incorporation of one or more peptides of interest, other than an Infectious Bursal Disease Virus sequence, within a Birnaviridae virus VP2 protein.
 2. The fusion protein according to claim 1 wherein VP2 is the VP2 protein of the Infectious Bursal Disease Virus or that shares at least 30% amino acid sequence homology with the VP2 protein of the Infectious Bursal Disease Virus.
 3. The fusion protein according to claim 1, wherein the VP2 protein is selected from the group consisting of: a full length 512 amino acid pVP2, or 456, or 452 or 441 amino acid fragment thereof, or comprises at least 400 amino acids of VP2.
 4. The fusion protein according to claim 1, wherein the peptide of interest is one or more amino acids long.
 5. (canceled)
 6. The fusion protein according to claim 1 , wherein the peptides of interest are incorporated within the VP2 P loop regions BC (Q219-G224), DE (R249-G254), FG (T283-D287) and HI (S315-Q324), and/or outside said VP2 P loop regions.
 7. The fusion protein according to of claim 1, wherein the incorporation of peptides of interest is by insertions or substitutions of VP2 protein sequences.
 8. The fusion protein according to claim 7 wherein the incorporation is by insertion.
 9. The fusion protein according to claim 7 wherein the incorporation is by substitution of at least one amino acid of the VP2 protein sequence by one or more peptides of interest.
 10. The fusion protein according to claim 9 wherein the substitution by the peptide of interest replaces at least one or more amino acids within VP2 P loop regions BC (Q219-G224), DE (R249-G254), FG (T283-D287) and HI (S315-Q324).
 11. The fusion protein according to claim 9, wherein the substitution by the amino acid or peptide of interest replaces VP2 protein sequences which are the same, or shorter, or longer length than the amino acid or peptide of interest.
 12. The fusion protein according to claim 1 wherein the peptides of interest are one or more lysines, or comprise at least one lysine.
 13. The fusion protein according to claim 12 wherein the one or more peptides of interest are chemically conjugated to one or more biological or chemical entities by chemical or biological conjugation.
 14. The fusion protein according to claim 1 wherein the fusion protein incorporates one or more peptides of interest at more than one site within VP2 protein at P loop regions and/or outside said VP2 P loop regions.
 15. The fusion protein according to claim 14 wherein the amino acid or peptides of interest are the same or different for each of the insertion or substitution sites.
 16. The fusion protein according to claim 1 wherein the peptide of interest is terminally fused either at its carboxy (C-) or amino (N-) terminal region, to a second amino acid or particular peptide of interest which may be the same or different to the peptides of interest inserted in the P loop region or outside said P loop regions.
 17. The fusion protein according to claim 1, wherein fusion proteins assemble to form Virus Like Particles (VLP).
 18. The fusion protein according to claim 17, wherein the Virus Like Particles present T=1 symmetry.
 19. A Virus Like Particle comprising an assembly of any of the fusion proteins as defined in claim
 1. 20. The Virus Like Particle comprising an assembly of the fusion proteins of claim 19 and a VP3 protein.
 21. The Virus Like Particle of claim 20 in which the VP3 protein is terminally fused, either at N- or C-terminal, to a peptide of interest which may be the same or different to those incorporated in VP2.
 22. The Virus Like Particle according to claim 19 which are obtained by the co-expression of pVP2 and VP3 from independent gene constructs or by the expression of constructs comprising the pVP2-VP4-VP3 polyprotein.
 23. The Virus Like Particle of claim 19 forms T=13.
 24. A nucleic acid encoding the fusion proteins defined in claim
 1. 25. A gene construct comprising the nucleic acid according to claim
 24. 26. An expression system comprising the gene construct according to claim 25 is operatively bound to transcription, and optionally translation, control elements.
 27. A host cell according to claim 26 containing the nucleic acid, the gene construct, or the expression system.
 28. (canceled)
 29. A process for the production of chimeric Virus Like Particles comprising culturing an organism or host cell according to claim 27 and, if desired, recovering said chimeric VLP.
 30. (canceled)
 31. A DNA vector library coding for Birnaviridae VP2 protein, or fusion proteins thereof, incorporating one or more cloning sites for the incorporation of DNA of interest leading any of the VP2 fusion proteins according to claim
 1. 32. A screening method for the location of the preferred insertion sites within VP2 proteins, or fusion proteins thereof, efficiently assembling into VLP, the method comprising: a. arranging, either individually or in pools, a set of any combination of DNA vectors as defined in claim 31; b. contacting the set of DNA vectors of a) with a DNA of interest coding for a peptide of interest, for the incorporation of said DNA of interest; c. transfecting, transforming or infecting host cells with the DNA expression vectors resulting from b) for the expression of chimeric VP2 fusion proteins; d. evaluating the VLP formation efficiency of host cells from c) and selecting clones expressing chimeric VLP with higher efficiencies; and e. determining or confirming the preferred insertion location of the DNA of interest.
 33. A random screening method for the location of preferred insertion or substitution sites within VP2 proteins, or fusion proteins thereof, efficiently assembling into VLP, the method comprising: a. constructing an expression vector containing a Birnaviridae VP2 DNA sequence or fusion protein thereof; b. contacting the DNA vectors of a) with a DNA sequence for its random incorporation throughout the VP2 sequence; c. transfecting, transforming or infecting host cells with the DNA expression vector library resulting from b) for the expression of chimeric VP2 fusion proteins; d. selecting transfectants from c) for higher VLP formation efficiencies; and e. determining the insertion location of the inserted DNA.
 34. The screening method according to claim 32, further comprising determining the preferred insertion sites for the fusion proteins of claim 1 to form chimeric VLP.
 35. Virus Like Particles obtained by the method as defined in claim
 32. 